A Phase III Study to Evaluate the Efficacy and Safety of AK111 in Subjects With Active Ankylosing Spondylitis

Inclusion Criteria: * Male or female subjects aged ≥18 years old. * Subjects with confirmed ankylosing spondylitis before screening. * During screening and before randomization, BASDAI score ≥ 4, total back pain score≥ 4. * Subjects received at least 2 kind of non-steroidal anti-inflammatory drugs (NSAIDs), prior to randomization with an inadequate response or failure to respond, or with contraindications or intolerance to the use of NSAIDs. * Subjects who are regularly taking NSAIDs, weak opioids or oral glucocorticoids(The daily dose should be ≤10mg prednisone or equivalent dose of glucocorticoid) as part of their AS therapy are required to be on a stable dose for at least 14 days before randomization. If the drug has been discontinued, at least 2 weeks washout period is required before randomization. * Subjects taking methotrexate (MTX) (≤25mg/week) or Sulfasalazine (≤3g/day) are allowed to continue their medication if started at least 12 weeks prior to baseline, with a stable dose for at least 4 weeks before randomization. If the drug has been discontinued, at least 4 weeks washout period is required before randomization. * Subjects who are able to understand and voluntarily sign the ICF and complete the study procedure. Exclusion Criteria: * Subjects with symptom of pain that affected the evaluation of efficacy. * Subjects with other inflammatory diseases or autoimmune diseases except Ankylosing spondylitis (AS). * Subjects who are using strong opioid analgesics. * Received glucocorticoid intramuscular or intravenous injection within 2 weeks prior to randomization; Received intraarticular or paraspinal glucocorticoid therapy within 4 weeks before randomization. * Received other antirheumatic drugs (except methotrexate, sulfasalazine), proprietary Chinese medicine or traditional Chinese medicine decoction, JAK inhibitor treatment for AS within 4 weeks before randomization. * Received Natalizumab or other B cell or T cell modulator in the 12 months prior to randomization. * Previous exposure to secukinumab, ixekizumab or any other biologic drug directly targeting IL-17 or IL-17 receptor. * Received multiple tumor necrosis factor α (TNF-α) inhibitors; The eluting period of biologics received before randomization is shorter than the protocol. * Participated in a clinical study of any other drug or medical device within 1 month (≤30 days) prior to randomization, or last received the investigational drug within 5 half-lives. * The presence of any other systemic disease or laboratory abnormalities that the investigator has judged unsuitable for clinical trials.