PD-1 Antibody Following Preoperative Chemoradiotherapy for Locally Advanced pMMR/MSS Rectal Cancer

Inclusion Criteria: * Patients who personally provided written consent for participation in the study * Treatment-naive patients with rectal cancer, in whom the inferior margin of the tumor was at a distance of 12 cm or less from the AV before CRT * Primary rectal cancer histopathologically confirmed to be adenocarcinoma * Clinical stage of T3,and T4 ,N any,M0,before CRT * Macroscopic radical resection could be feasible, based on diagnostic imaging before CRT * Patients with the ECOG performance status of 0 or 1 at the time of enrollment * Patients without distant metastasis on the imaging test before CRT * Preoperative biopsy of tumor immunohistochemistry TPS\>1% or CPS\>1 * Life expectancy of greater than 2 years * No signs of intestinal obstruction; or the obstruction has been relieved after the proximal colostomy operation * Hematology: WBC\>4000/mm3; PLT\>100000/mm3; Hb\>10g/dL * Liver function: SGOT and SGPT are less than 1.5 times the normal value; bilirubin is less than 1.5mg/dL * Renal function: creatinine \<1.8mg/dL Others: non-pregnant or breast-feeding women; no other malignant diseases (except for non-melanoma or cervical carcinoma in situ) within 5 years or during the same period; no mental illness that causes the inability to obtain informed consent; no other serious diseases that can shorten the survival time disease. * Have not received rectal surgery in the past; * Have not received chemotherapy or radiotherapy in the past; * Have not received biological treatment in the past; * Past endocrine therapy: unlimited. Exclusion Criteria: * Rectal cancer with unstable microsatellite (MSI or dMMR); * Preoperative biopsy of tumor immunohistochemistry TPS≤1% or CPS≤1 * Known history of human immunodeficiency virus (HIV) or chronic hepatitis B or C (high copy viral DNA); * Autoimmune diseases; * Other active clinical serious infections (\>NCI-CTC version 3.0); * Patients in clinical phase I; * There is evidence that there is distant metastasis before surgery; * Cachexia, decompensation of organ function; * Have a history of pelvic or abdominal radiotherapy; * Multiple primary cancers; * Patients who need treatment for seizures (such as steroids or anti-epileptic treatment); * Have a known additional malignant tumors within 5 years. Exceptions include basal cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer; * Chronic inflammatory bowel disease, intestinal obstruction; * Drug abuse and medical, psychological or social conditions may interfere with patients' participation in research or have an impact on the evaluation of research results; * Known or suspected to be allergic to the study drug or to any drug given in connection with this test; * Any unstable conditions or situations that may endanger patient safety and compliance