Objective To explore the application of 3.0 T MRI in the early diagnosis and preoperative T staging of rectal cancer. Methods This study selected 120 cases cancer patients which proved by endoscopic biopsy and accepted full abdomen MRI plain scan and contrast enhancemen before surgery from 2012 December to 2014 September in our hospital, the surgical operation and histopathological examination were within one week after the MR scan. To analyzed the clinical value of 3.0 T MRI in the early diagnosis and preoperative T staging of rectal cancer by the gold standard of operation. Results 120 cases of patients with rectal cancer, MRI T stage showed: 8 cases of stage T1, 37 cases were T2 stage, 54 cases were T3 stage and 21 cases were T4 stage. Pathological T stage showed: 8 cases were T1 stage, 35 cases were T2 stage, 56 cases were T3 stage and 21 cases were T4 stage. The Kappa test shows, both of them have high consistency (Kappa=0.710, P=0), the diagnostic accuracy was 86.67% (104/120); the correct diagnostic rate of T3 staging rectal carcinoma by 3.0 T MRI was 92.45%, and was the highest; the correct diagnostic rate of T1 staging rectal carcinoma by 3.0 T MRI was 75%, and was the lowest; the specificity of T3 staging by MRI is the highest (98.35%), sensitivity, negative predictive diagnosis of stage T4 rate of 100%; the results of 3.0MRI and pathological examination showed edge of distance and anal margin of the tumor respectively (4.20±0.51) cm, (4.21±0.45) cm, there was no statistically significant difference. Conclusion 3.0 T MRI can accurately diagnose early rectal carcinoma and T staging, which has important significance on preoperative evaluation and prognosis.