Objective To evaluate the diagnostic value of MRI with water-bag in rectum for the preoperative staging of rectal cancer. Methods Preoperative MRI was performed in 80 patients with pathology confirmed rectal cancer. 40 of them were randomly assigned for MRI with water-bag in rectum, and the last 40 was assigned for conventional MRI. The diagnostic value of tumor (T) and lymph node (N) staging were analyzed, and the results were compared with postoperative pathology results. The diagnostic accuracy of these two methods was also compared. Results The results of T staging of MRI with water-bag in rectum were in good agreement with the histopathology results (P<0.01), and the results of N staging were in common agreement (P<0.01), total T and N staging accuracy were 90% and 77.5%, the results of T and N staging of conventional MRI were in common agreement with the histopathology results (P<0.01), total T and N staging accuracy were 72.5% and 80%. There was statistically significant difference (P<0.05) between T-staging results of MRI with water-bag in rectum and conventional MRI, and no significant difference (P＞0.05) between N-staging results. Conclusion Preoperative MRI with water-bag in rectum have certain value for T-staging of rectal cancer, and shows no significant difference in N-staging with conventional MRI.