Objective To explore significance of clinical application of MRI 3.0T in the diagnosis and preoperative TN staging of rectal cancer. Methods 68 cases of rectal cancer diagnosed by pathologic biopsy in our hospital were selected, 3.0T MRI examination was used to diagnose and preoperative TN staging of the patients, the results were compared with surgical pathologic findings. Results Comparing the preoperative and surgical pathologic T staging by 3.0T MRI, after Kappa test, the consistency is high (k=0.61, P<0.05), the diagnostic accuracy was 89.71%, diagnosis accuracy, specificity and positive predictive value of T1 staging were the highest, diagnostic sensitivity of T2 staging was the highest, and negative predictive value of T4 staging was highest. Comparing the preoperative and surgical pathologic N staging by 3.0T MRI, after Kappa test, the consistency is high (k=0.54, P<0.05), the diagnostic accuracy was 69.12%, diagnosis accuracy, specificity, positive predictive value and negative predictive value of N2 were the highest, diagnostic sensitivity of N0 staging was the highest; there was no significant difference in the distance from the lower margin of the tumor to anal verge of 3.0T MRI and pathological results (P>0.05). Conclusion The accurate rate of MRI 3.0T in the diagnosis and preoperative TN staging of rectal cancer was high, can offer bases on clinical operation, is one ideal inspection method and worth spreading and applying.