Purpose: To describe the appearance of the bone marrow distribution changes adjacent to the sacroiliac joint（SIJ）, including both AS-related and normal changes on MRI, and to highlight the potential MR features that might be misleading. Methods: The study included 31 AS patients aged from 14 to 46 years old and 89 normal adults aged from 17 to 87 years old. A variety of sequences includes axial and coronal T1WI ,STIR were scanned both in AS patients and the healthy ones. The fat fraction value(FF value)were measured by two Senior radiologists. Results: The FF of SIJ bone marrow were increased with the healthier’ age. There was no significant difference between the middle-aged group and the elderly group（P=0.2212）. The difference between the adolescent group and the middle-aged group was statistically significant（P < 0.001）; in the chronic stage and the late stage，the SIJ FF of patients with AS was significantly greater than the ones at active stage （P < 0.001）, the difference was statistically significant (Table 1); the difference between chronic stage and late stage was not statistically significant（P = 0.6362）. In addition, there was no significant difference in FF between the SIJ in chronic AS patients and the degenerated SIJ in patients, but the former usually showed localized deposition and the latter showed diffuse distribution. Conclusion: By studying the bone marrow distribution changes adjacent to the sacroiliac joints on MRI, it’s helpful to diagnose AS and reduce the misdiagnosis or miss diagnosis.