Objective To investigate the diagnostic value of magnetic resonance imaging (MRI) in the preoperative TN staging of rectal cancer. Methods 68 cases of rectal cancer treated in our hospital from February 2016 to March 2017 were selected, MRI scan was performed before the operation, and the scanning sequence included T2WI and magnetic resonance diffusion weighted imaging (DWI) (b=0,1000s/mm2 ). The preoperation TN staging of T2WI and postoperative pathological results were analyzed, the apparent diffusion coefficient (ADC) of different T staging lesions (ADC) was measured by DWI. Results The postoperative pathological T staging: 10 cases of ≤T2, 39 cases of T3, 19 cases of T4, postoperative pathological N staging: 45 cases of N0, 17 cases of N1, 6 cases of N2. The Kappa value of T staging of MRI before operation and pathological results was=0.642, P<0.05, and the accuracy rate of T staging of MRI before operation was 79.41%. The Kappa value of N staging of MRI before operation and pathological results was=0.244, P<0.05, and the accuracy rate of N staging of MRI before operation was 57.35%. The ADC value of the T1+2 phase was (1.104±0.122)×10-3mm2 /s, which was significantly higher than that of T3 and T4 (P<0.05). Conclusion MRI has a good application value in the diagnosis of preoperative T staging of rectal cancer, and its N stage diagnosis accuracy is low. ADC value plays a certain role in preoperative T stage diagnosis.