Forty-four patients with AS underwent MRI including DKI to evaluate the disease activity of AS. Four disease activity states were chosen: inactive disease and moderate, high, and very high disease activity. The D and ADC values of sacroiliac joints in the active group were significantly higher, while the K value was significantly lower than those of the inactive group. The D value performed best in distinguishing different activity grades. The D and ADC values correlated positively, while the K value correlated negatively, with activity grade significantly. DKI of sacroiliac joints might be useful to evaluate the disease activity of AS.