Objective To evaluate the value of diffusion weighted imaging (DWI)in diagnosing active sacroiliitis of ankylosing spondylitis(AS)Methods 63 cases with AS confirmed clinically were included.The patients were evaluated by Bath ankylosing spondylitis disease activity index (BASDAI)score and examined by MRI with oblique coronal DWI and STIR sequence in the sacroili-ac joint.The ADC value and Spondyloarthritis Research Consortium of Canada (SPARCC)score were measured in all patients.Cor-relative analysis was performed among BASDAI score,SPARCC score and ADC value.Patients were divided into activity and stability group according to the condition,which were compared with the ADC features of the sacroiliac joint in healthy volunteers.Results The Spearman correlation analysis showed that in patients SPARCC score and ADC values (r =0.781,P <0.001 ),BASDAI score and ADC values (r=0.854,P <0.001),SPARCC score and BASDAI score (r =0.805,P <0.001)were positively correlated.The median ADC values in the sacroiliac joint of activity group (0.704×10 -3 mm2/s)was higher than that in the stable group(0.543× 10 -3 mm2/ s )and the control group (0.431×10 -3 mm2/ s).The ADC values between the activity、stability and control group were statistically significant differences.Conclusion SPARCC score and DWI are important indexes for MRI evaluation of sacroiliac ar-thritis activity.DWI is a fast imaging sequence,can be quantified by measuring the ADC value of the lesion area,and is the ideal choice for sacroiliac joint imaging.