This is a randomized, double-blind, placebo-controlled, multi-center phase III clinical study to evaluate the efficacy and safety of AK111 in the treatment of subjects with active ankylosing spondylitis.

The study consists of 3 parts. Part 1 is screening period, Part 2 is Placebo control period and part 3 is Long term treatment follow-up period. The research period is 61 weeks in total.

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.