This study is a 12-week, randomized, placebo and positive drug controlled clinical trial to investigate whether tripterygium capsule is effective and safe in treating patients with ankylosing spondylitis. The investigators expect that the tripterygium capsule is more effective than placebo and has few adverse effects.

Inclusion Criteria:

1. Fulfill the ESSG criteria for SpA and not fulfill any criteria for the subtype including ankylosing spondylitis, psoriatic arthritis, reactive arthritis and inflammatory bowel disease associated arthritis.
2. Disease is in active status defined by BASDAI>=40mm.
3. DMARDs administration must be suspended for at least 4 weeks before screening. NSAIDs dose must be stable for at least 4 weeks before screening.

Exclusion Criteria:

1. Intra-articular injection of glucocorticoid within 3 months.
2. Previous history of heart failure, multiple sclerosis, chronic obstructive pulmonary disease, recurrent infection, lymphoma or other tumors.
3. Accompanied by fibromyalgia syndrome or other rheumatic diseases.
4. Female of pregnancy or breast-feeding.
5. Poor compliance or with mental diseases.