The study was using fat fraction (FF) to investigate the fat deposition of sacroiliitis. 3T MRI with mDIXON Quant technique was performed. All patients were divided into three groups: simple edema group, edema-fat deposition group and simple fat deposition group. The FF of both edema and fat deposition areas were obtained on FF image. The FF of edema areas was higher in edema-fat deposition group than simple edema group. The FF of fat deposition areas was higher in simple fat deposition group than edema-fat deposition group. mDIXON Quant technique could be used for assessing the situation of the disease.