Objective In this study，the difference of the?depiction?rate and richness of synovial blood flow in the sacroiliac joint（SIJ） of patients with active ankylosing spondylitis（AS） were compared between the two models of blood flow，superb microvascular imaging（SMI）and power doppler ultrasound（PDUS），explored the clinical value of SMI in evaluating SIJ inflammation in patients with active AS.Methods Two blood?flow?patterns?of SMI and PDUS were used to examine 60 sacroiliac?joints on both sides of 30?healthy?volunteers（the control group） and 30 AS?patients（the?patient?group） respectively in this study，the differences of the?depiction?rate of blood flow、richness of synovial blood flow were compared between the two groups.Kappa tested the consistency of diagnostic results of the two models, and ROC curve calculated the diagnostic efficiency of the two methods.Results The difference of blood flow display rate in two blood flow models was only statistically significant in AS patients （P=0.048），There were Statistical differences in the blood flow richness of sacroiliac joint of the AS patients and healthy?volunteers of the two blood?flow?patterns（P＜0.05）.The consistency of blood flow classification under the two blood flow patterns was better in the control group than patient? group（control group kappa=0.761，patient group kappa=0.686，P＜0.001）.There was no significant difference in diagnostic?efficacy?of?the two methods with blood flow richness for?diagnosis?of sacro-iliitis?in AS patients（SMI ?ROC area 0. 783，PDUS?ROC?area?0.775）. Conclusion The blood flow richness of SMI and PDUS can provide an important reference for the diagnosis of sacro-iliitis in AS patients and evaluation of their activity.The diagnostic efficacy of SMI and PDUS in the evaluation of sacro-iliitis in AS patients is basically the same.SMI can increase the display rate and elevate the classification of sacroiliac joint synovial blood flow，the blood flow display is clearer compared with PDUS，SMI has an advantage in evaluating the activity of sacro-iliitis in AS patients.