Ankylosing Spondylitis (AS) is a chronic progressive disease that affects the sacroiliac joints, spondylolisthesis, paraspinal soft tissues, and peripheral joints, and may be associated with extra-articular manifestations. AS is the prototype of spondylolisthesis or primary AS, while other spondylolisthesis complicated by sacroiliac arthritis is secondary AS, usually referred to as the former and referred to in this guideline. The prevalence of AS varies from country to country, with 0.05% to 0.2% in Japan and 0.26% in China in a preliminary survey. It was previously thought that the disease was more common in men, with a male to female ratio of 10.6:1; now the male to female ratio is reported to be 5:1, except that the onset of the disease is slower and less severe in women. The age of onset is usually between 13 and 31 years, and it is rare after 30 years of age and before 8 years of age. The etiology of AS is unknown. Epidemiological investigations have revealed that genetic and environmental factors play a role in the development of the disease. It has been shown that the onset of AS is closely related to HLA-B27 (hereafter referred to as B27), and there is a clear familial tendency to develop the disease. The rate of B27 positivity in the normal population varies greatly by race and region, e.g., 4%-13% in Caucasians in Europe and 2%-7% in China, but the rate of B27 positivity in AS patients in China is 91%. Other data show that the prevalence of AS is 0.1% in the general population, 4% in the family line of AS patients, and up to 11%-25% in the first-degree relatives of B27-positive AS patients, suggesting an increased risk of AS in B27-positive patients or those with a family history of AS. However, approximately 80% of B27-positive individuals do not develop AS, and approximately 10% of AS patients are B27-negative, suggesting that other factors are involved in the pathogenesis, such as intestinal bacteria and intestinal inflammation. One of the pathological hallmarks and early manifestations of AS is sacroiliac arthritis. The typical manifestation of spinal involvement in advanced stages is a bamboo-like spine. Synovitis of peripheral joints is histologically indistinguishable from rheumatoid arthritis. Terminal tendinopathy is one of the features of the disease. Focal mesangial necrosis of the aortic root can cause annular dilatation of the aorta as well as shortening and thickening of the aortic valve cusps, leading to incomplete closure of the aortic valve.