Based on the rock nonlinear elastic-plastic theory and with the CaoJiawan station of Chongqin Rail Transit line 6 as an engineering case, makes numerical analysis and mechanical properties study on the channel cross section of shallow tunnel. Establishes a three-dimensional model of the complicated cross section of shallow tunnel by the finite element numerical software FLAC3D, studies the vault sedimentation and surrounding rock convergence of three different sections of the tunnel crossing as well as the variable characteristics of internal force of primary support steel, and compares with the data from field monitoring and measuring. The results show that the interrelationship exists in main channel cross section and secondary channel cross section; the vault and vacancy shift of three sections obviously changes; the initial branch bars are subjected to stress influences in the cross tunnel vault and arch area.