A Phase 3 Study to Investigate the Efficacy and Safety of SHR0302 With Moderately to Severely Active Ulcerative Colitis

Inclusion Criteria: Inclusion Criteria for Part 1 1. Male or female subjects must be at least at ≥18 and ≤75 years of age 2. Subject has at least a three-month history of Ulcerative Colitis diagnosis at baseline. 3. Subject has active Ulcerative Colitis with a 9-point modified Mayo score of ≥ 5 at baseline, with an endoscopic subscore of ≥ 2 4. Subject is deemed by the physician as having inadequate response, loss of response or intolerance to at least one conventional treatment (oral 5-ASA, immunosuppressants or corticosteroids), or was previously exposed to anti-TNF therapy (e.g., infliximab, adalimumab) or other biological treatment (e.g., vedolizumab) having Discontinued the treatment for: * Infliximab: a minimum of 8 weeks prior to baseline. * Adalimumab: a minimum of 10 weeks prior to baseline. * Ustekinumab: a minimum of 14 weeks prior to baseline. * Vedolizumab: a minimum of 17 weeks prior to baseline. Inclusion Criteria for Part 2 1. Subject has completed Part 1 and achieved clinical response at week 8 Inclusion Criteria for Part 3 1\. Subject has completed the 8-week Part 1 and was classified as not meeting clinical response criteria. OR Subject has discontinued treatment early in the Maintenance phase due to disease worsening OR Subject has completed the Maintenance phase. Study Exclusion Criteria for Part 1 Exclusion Criteria: 1. Subject has a diagnosis of indeterminate colitis, or clinical findings suggestive of Crohn'sDisease. 2. Subject with Ulcerative Colitis, which is confined to a proctitis (distal 15 cm or less). 3. Treatment naïve subject diagnosed with Ulcerative Colitis (without previous exposure to any of the following therapies for UC treatment: oral 5-ASA, corticosteroids, immunosuppressants, or biological treatments). 4. Subject is displaying clinical signs of ischemic colitis, fulminant colitis or toxic megacolon. 5. Subject had previous surgery as a treatment for Ulcerative Colitis or likely to require surgery during the study period. 6. Subject has evidence of pathogenic bowel infection. Subjects had Clostridium difficile or other intestinal infection within 30 days of screening endoscopy or test positive at screening for C.difficile toxin or other intestinal pathogens. 7. Subject currently has or has a history of active tuberculosis (TB) or latent TB infection. 8. Subject is receiving any of the following therapies: * Azathioprine/6-mercaptopurine, methotrexate, thalidomide within 7 days prior to baseline. * Cyclosporine, mycophenolate, tacrolimus within 4 weeks prior to baseline. * Interferon therapy within 8 weeks prior to baseline. * Intravenous corticosteroids or rectally administered formulation of corticosteroids or 5- ASA within 2 weeks prior to baseline. 9. Subject had any prior treatment with lymphocyte-depleting agents/therapies (such as CamPath® \[alemtuzumab\], alkylating agents \[e.g., cyclophosphamide or chlorambucil\], total lymphoid irradiation, etc.). Subjects who have received rituximab or other selective B lymphocyte depleting agents are eligible if they have not received such therapy for at least 1 year prior to baseline. 10. Subject has previously received JAK inhibitors, such as tofacitinib, baricitinib, upadacitinib, filgotinib. 11. Subject with evidence of clinically relevant laboratory abnormalities which may affect subject safety or interpretation of study results at screening 12. Subject has a screening 12-lead ECG that demonstrates clinically relevant abnormalities 13. Subject currently has or had: * A clinically significant infection within 1 month of baseline (e.g., those requiring hospitalization or parenteral antimicrobial therapy or have opportunistic infections). * A history of more than one episode of herpes zoster, or disseminated zoster (single episode). * Any infection otherwise judged by the investigator to have the potential for exacerbation by participation in the study. * Any infection requiring antimicrobial therapy within 2 weeks of screening. 14. Subject has current immunization with any live virus vaccine or history of immunization with any live virus vaccine within 8 weeks of baseline. 15. Subject with a first-degree relative with a hereditary immunodeficiency. 16. Subject with a history of any lymphoproliferative disorder (such as EBV-related lymphoproliferative disorder, as reported in some subjects on other immunosuppressive drugs), history of lymphoma, leukemia, multiple myeloma, or signs and symptoms that are suggestive of current lymphatic disease. 17. Subject has any condition possibly affecting oral drug absorption e.g., gastrectomy, or clinically significant diabetic gastroenteropathy, or certain types of bariatric surgery such as gastric bypass. (Procedures such as gastric banding, gastric balloon that simply divide stomach into separate chambers, are NOT exclusionary.) Subject has undergone significant trauma or major surgery within 4 weeks of baseline. 18. Women who are pregnant or lactating, or planning pregnancy while enrolled in the study. Male who plan to donate sperm during the study and within 30 days after the last dose of study drug. 19. Subject who has a history of alcohol or drug abuse with less than 6 months of abstinence prior to baseline that in the opinion of the investigator will preclude participation in the study. 20. Subject with malignancies or with a history of malignancies with exception of adequately treated or excised non-metastatic basal cell or squamous cell cancer of the skin. 21. Subject infected with human immunodeficiency virus (HIV) or hepatitis B or C viruses. 22. Subject has received any investigational drug or device within 3 months, or 5 half-lives (if known) prior to baseline. 23. Subject is receiving or expected to receive prohibited concomitant medication(s) in the 4 weeks prior to the first dose of study drug and through follow-up visit. 24. Any other condition which in the opinion of the investigator would make the subject unsuitable for inclusion in the study. 25. Subject with historical or current evidence of clinically significant cardiovascular, neurological, psychiatric, renal, hepatic, immunological, gastrointestinal, urogenital, nervous system, musculoskeletal, skin, sensory, endocrine (including uncontrolled diabetes or thyroid disease), or hematological abnormalities that are uncontrolled. Significant is defined as any disease that, in the opinion of the Investigator, would put the safety of the subject at risk through participation, or which would affect the efficacy or safety analysis if the disease/condition exacerbated during the study. 26. Subject with a history of thromboembolic events, including deep vein thromboses (DVT), pulmonary embolism (PE), and those with known inherited conditions that predispose to hypercoagulability. Study Exclusion Criteria for Parts 2 and 3 1. Subject with any clinically significant condition at the end of 8-week Induction treatment from Part 1 Induction phase, and Part 2 Maintenance Phase that in the opinion of investigator, would put the safety of the subject at risk through participation, or which would affect the efficacy or safety analysis. 2. Subject who, in the opinion of the investigator or sponsor, is unlikely to be cooperative or able to comply with study procedures, or any other condition which in the opinion of the investigator would make the subject unsuitable for inclusion.