Geological disasters such as landslides occur frequently in eastern Sichuan, and with the continuous expansion of urban space, it is inevitable for urban road mountain tunnel openings to cross bedding slopes. To find a safe and rational design method for the side slope of the tunnel opening through bedding slope, a supplementary investigation and site survey are conducted on the side slop sliding of a road near a tunnel opening, the mechanism of slope sliding is analyzed. Then, the transfer coefficient method and strength reduction method are used to invert and verify the shear strength parameters of the sliding surface soil. Finally, a three-dimensional finite element numerical simulation of the tunnel entrance slope is established to carry out the stability evaluation of the tunnel entrance slope. The results show the following: (1) The safety coefficient obtained by the transfer coefficient inversion method is close to that obtained by the strength reduction method, indicating that the inversion results of landslide parameters can be applied to the three-dimensional numerical model calculation of the side slope of the tunnel entrance. (2) The forces at the side elevation slope of the tunnel entrance are complicated, and the effect of tunnel structure and support on the side elevation slope is not considered in the numerical simulation stability calculation, which has limited impact on the stability calculation results but can enhance the safety measures at the entrance. (3) The ground deformation monitoring results of subsequent construction are close to the calculated data of the three-dimensional numerical model, indicating that the landslide inversion parameters can reasonably simulate the stability calculation of the real side slope along the bed, and verify the rationality, accuracy, and reliability of the design and construction of the side slope of the tunnel entrance and the finite element simulation method.