CT of the sacroiliac joint is one of the main bases for the diagnosis of ankylosing spondylitis, so CT can detect ankylosing spondylitis. Sacroiliac joint is the earliest part of ankylosing spondylitis, so as long as there is a suspicion of ankylosing spondylitis, you can do X-ray and CT of sacroiliac joint.CT can be clearer and earlier than X-ray to find out the pathological changes of the sacroiliac joint and to judge the staging of the disease. In early stage, the edge of sacroiliac joint is blurred and slightly dense, and the joint space is widened. In the middle stage, the gap between sacroiliac joints is narrowed, the bone at the edge of the joints is destroyed and interlaced with dense hyperplasia, which is jagged, and the dense band on the iliac side is widened. In the late stage, the sacroiliac joint space and dense band disappeared, and the bone trabeculae passed through with bony ankylosis.