Abstract: Objective: To analyze the clinical characteristics of diagnosis and therapy of pyogenic sacroiliitis. Methods: 4 cases were analyzed retrospectively and reported;and related literatures were reviewed. Results: All patients had fever and pain in the unilateral sacroiliac joint region or buttock;all patients were misdiagnosed as ankylosing spondylitis.Serological inflammatory indexes were normal or slightly elevated.Imageology showed obfuscated interspace of the sacroiliac joint and swelling soft tissue.Microbiology of 2 females and 1 male were positive.All patients were treated with antibiotic and their symptoms were relieved. Conclusion: MRI imageology should be used early when patients are suspected of pyogenic sacroiliitis.Early diagnosis and treatment will produce good clinical therapeutic effect.