This is a multi-center, double-blind, randomized, parallel-group, placebo-controlled study to evaluate the efficacy and safety of ENIA11 in patients with ankylosing spondylitis. The study period for each patient will be 27 weeks, during which the patient will undergo screening for up to 21 days, and followed by treatment of 24 weeks and follow up period of 2 weeks. Each patient will be required to make a total of 9 visits.