Ankylosing spondylitis is a type of polyarthritis characterized by the progressive upward spread of inflammation from the sacroiliac joints to the spinal joints, resulting in bony ankylosis. The lesions are predominantly in the trunk joints and may also affect the hip joints near the trunk, but rarely in the small joints of the extremities. Synovial hypertrophy and erosion of the articular cartilage surfaces are mild, and bone resorption and joint dislocation rarely occur, but the ossification of the joint capsule and ligaments is more prominent, and the calcification and ossification of the articular cartilage surfaces make it very easy for osseous ankylosis to occur. Inflammatory granulation tissue in the union can both erode the osteophytes in the union and spread into the ligaments, tendons, and joint capsule. During the process of tissue repair, too much new bone is produced, which is not only sufficient to fill the osteophyte defect, but also transitions to the nearby ligaments, tendons, and joint capsule, forming a ligamentous osteophyte. This proliferation and developmental outcome is an important cause of osseous ankylosis of the joint. This change is mostly seen in the hip joint, but also in the intervertebral discs, interarticular joints, sacroiliac joints, sciatic tuberosity, and pubic tuberosity. The disease is most often seen in young men aged 15 to 30 years old, and there is mostly a family history of the disease. In the early stage, the lumbosacral region, hip and back are sore and stiff, the anterior protrusion of the lumbar vertebrae disappears, and the lumbar spine cannot be raised and can only walk semi-prone. The lesions mostly start in the sacroiliac joint, gradually spread upward to the cervical spine, and finally lead to spinal ankylosis, and the large joints of the extremities can also be involved. About 80% of patients have alternating attacks and remissions, and the duration of the disease can be several years or decades. In about 20% of patients, the onset of the disease is rapid, with systemic symptoms such as high fever, etc. X-rays show blurred and rarely dense margins of the sacroiliac joint in the early stage, with widening of the joint space. In the middle stage, the joint space is narrowed, and the joint edges are interspersed with osteophytes and erosions in a jagged shape. In the late stage, the joint gap disappears. In the early stage, only osteoporosis is seen in the spine. In the middle and late stage, small bone spurs, square vertebrae, small joint fusion, and calcification of joint capsule and ligaments appear. Spinal ankylosis in the form of “bamboo joints” and sacroiliac joint changes are one of the main bases for the diagnosis of this disease.