Ankylosing spondylitis (AS) is a disease in which inflammation of the sacroiliac joints and spinal attachment points is the main symptom. Sacroiliac arthritis is a characteristic sign of ankylosing spondylitis, but why is ankylosing spondylitis prone to invade the sacroiliac joints? The reasons may be as follows: 1, related to the anatomical position: sacroiliac joint is a micro-movement joint formed by the interconnection of the sacrum and the auricular joint surface of the iliac bone, with little mobility, and is a necessary channel for gravity transmission, while the joint surface of the sacroiliac joint is uneven in adulthood, with a staggered shape, and the joint capsule is close to the joint surface, although there are strong ligaments to make the joint firmly connected, but due to the structural characteristics of the sacroiliac joint, it is easy to suffer damage from gravity and external force. 2, related to the characteristics of the tissue structure: the ligaments around the sacroiliac joint are more dense, and it is adjacent to the rectum, bladder and uterus (female), prostate (male) and other capillary-rich organs, the first three are cavernous organs connected with the outside world, increasing the pathway for external bacterial infection. 3.Related to infection: the anatomical position and tissue structure of sacroiliac joint determines its higher chance of infection. There is rich blood and lymphatic circulation near sacroiliac joint, which supplies nutrition on one hand, but creates good conditions for bacterial breeding and reproduction on the other hand. 4, related to immunity: from ankylosing spondylitis and HLA-B27 antigen is highly correlated with the consideration of whether there are substances on the sacroiliac joint related to HLA-B27 antigen, or whether there are receptors on the sacroiliac joint related to the immune response to ankylosing spondylitis, this is only a deduction, and further research is needed in the future.