In order to analyze the problems of lining deformation, cracking, and even tunnel collapse caused by the presence of weak interlayers in the surrounding rock of highway tunnels in mountainous areas under the orthogonal system, based on the theory of tensile shear strength reduction method and FLAC3D finite difference software, this paper takes the planned expansion of a certain tunnel slope as the engineering background, and studies the changes in the stability and safety coefficient of the tunnel slope surrounding rock with different slope slopes and tunnel burial depths. The results indicate that as the slope gradient continues to increase, the maximum shear strain value and distribution range of the surrounding rock gradually increase, the plastic zone range continues to increase, and the safety factor of the surrounding rock stability continues to decrease. As the depth of the tunnel increases, the maximum shear strain value of the surrounding rock first increases, then decreases, and finally increases. However, the ranges of the shear strain and plastic zone continues to increase, and the safety factor of the surrounding rock stability shows a first decrease and then increase. When there is no weak interlayer, the maximum safety factor is calculated. When there is a weak interlayer, the safety factor calculated using the asynchronous reduction method is higher than that using the synchronous reduction method.