Objective：This study aims to describe the different appearances of bone marrow distribution changes adjacent to the sacroiliac joint in both ankylosing spondylitis（AS） patients and volunteer control without seronegative spondy-loarthropathy（SPA） at different ages，and to highlight the potential MR imaging features that may lead to misdiagnosis. Methods：The study included 31 AS patients aged from 14 to 46 years old and 89 volunteers without SPA aged from 17 to 87 years old. AS patients were divided into 3 groups：active stage AS patients were divided group，chronic stage group and endstage group. Volunteers were divided into three groups according to their age：40 years old（adolescent group），≥40~60 years old（middle-aged group）and ≥60 years old（old group）. Distribution of yellow bone marrow in sacroiliac joint was measured after routine MR scanning，and the percentage of yellow bone marrow in sacroiliac joint（fat faction，FF） was calculated. Results：FF value in sacroiliac joint of AS patients in chronic and end-stage was significantly higher than that in active stage，and the differences were was statistically significant（P < 0.001）. There was no significant difference in FF between chronic and end-stage（P=0.636）. In addition，there was no significant difference in the distribution of yellow bone marrow in sacroiliac joint between patients with AS in chronic stage and volunteers with degenerated sacroiliac joint，but the former usually showed localized deposition，while the latter showed diffuse distribution. With the increase of age，the content of yellow bone marrow in sacroiliac joint increased gradually; there was no significant difference in FF value between middle-aged group and old group（P=0.221）; there were significant differences in FF value between adolescent group and middle-aged group or old group（P < 0.001）。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.