A total of 94 patients with rectal cancer confirmed by pathology were selected，and all patients were examined by 3D-ERUS and magnetic resonance imaging （MRI）. Receiver operating characteristic（ROC） curve was used to evaluate the results of 3D-ERUS and MRI in the diagnosis of lymph node metastasis；the precision，recall and F1 score were used to evaluate the results of 3D-ERUS and MRI in the diagnosis of N staging.Kappa coefficient was used to evaluate the consistency between the results of 3D-ERUS and MRI in the diagnosis of lymph node metastasis and N staging and the pathological results. Chi square test was used to compare the differences between the ultrasound features and clinicopathologic factors and lymph node metastasis of rectal cancer，and the factors with statistical differences were analyzed by multivariate Logistic regression.

The accuracy of 3D-ERUS in the diagnosis of lymph node metastasis was 80.85%（Kappa=0.615，P<0.01）， respectively， and the area under curve（AUC） was 0.831； the accuracy of MRI in the diagnosis of lymph node metastasis was 70.21%（Kappa=0.415，P<0.01），respectively， and the AUC was 0.728.The AUC of 3D-ERUS and MRI had statistical difference（Z=2.039， P=0.041），and there was no statistical difference in the accuracy of diagnosis between them（χ²=2.33，P=0.126）.The overall accuracies of 3D-ERUS and MRI in distinguishing N staging were 74.47% and 63.44%，and there was no statistical difference（χ²=2.17，P=0.141）. The univariate analysis results showed that the related factors of lymph node metastasis were pathological T stage，carcinoembryonic antigen（CEA）， histological differentiation and 3D-ERUS T stage（uT-stage）.The multivariate analysis results showed that the main risk factor of lymph node metastasis was pathological T stage（P<0.01）；compared with stage T_is-T₂ rectal cancer，stage T₄ rectal cancer［odds ratio（OR）=12.000，95% confidence interval（CI）：3.141－45.839］ had the higher risk in the occurrence of lymph node metastasis.

3D-ERUS has high accuracy and clinical value in the preoperative diagnosis of N staging of middle and lower rectal cancer， and pathological T4 rectal cancer has the highest risk of perienteral lymph node metastasis.