Abstract: According to the existing codes, when the thickness of a layer is less than 5 m, the shear strength parameters of horizontal interbedded sandstone and mudstone are the same as those of mudstone. However, for different project scenes and thickness ratios, the shear strength parameters have not been further distinguished. The ratio of mudstone thickness to sandstone thickness is introduced in this paper and the tri-axial numerical test model with different thickness ratios is established by using FLAC3D. The effect of the shear strength parameters and deformation characteristics of the horizontal interbedded sandstone and mudstone rock mass is explored. The results show that when the layer thickness ratio is less than 0.75, the smaller the layer thickness ratio is, the greater the cohesion is. This reflects the increasing effect of sandstone on the cohesion of the rock mass. In this interval, the cohesion of the horizontal interbedded sandstone and mudstone rock mass can be valued at 1 to 4 times the mudstone cohesion. In the same way, the internal friction angle in the rock mass is not objective only according to the value of mudstone, but the law of its variation with layer thickness ratio is not obvious. When the layer thickness ratio is greater than 0.75, it is feasible that the shear strength parameters of the horizontal interbedded sandstone and mudstone rock mass are selected based on the mudstone value. When the layer thickness ratio is constant, the layer thickness has obvious influence on the shear strength parameters of the horizontal interbedded sandstone and mudstone rock mass. When the layer thickness ratio is less than 0.2, the cohesive force of the interbeded rock mass varies greatly with the layer thickness, which also indicates that the value of the mudstone alone is not objective. When the layer thickness ratio is greater than 0.2, the shear strength parameter varies little with the layer thickness and can be selected based on the mudstone value. When the layer thickness ratio is less than 0.75, the internal friction angle of the interbeded rock mass varies greatly with the layer thickness, which also indicates that the value of the mudstone alone is not objective. When the ratio of layer thickness to thickness is greater than 0.75, the friction angle within the rock mass changes little with the layer thickness, which can be chosen according to the mudstone value.