Based on the direct shear test theory, mechanical properties of steel and sandstone mixed with mudstone interface are studied by experiments. In the tests, steel plate is made from Q235 steel, soil materials of sandstone mixed with mudstone is composed of weak weathered sandstone and mudstone after crushing, soil material weights ratio of 8∶2 prepared by sandstone and mudstone with 5 different particle grading curves. The tests results show that the shear stress-displacement curve of steel and sandstone mixed with mudstone interface can be used by hyperbolic model fitting; shear strength can be described by the Mohr-Coulomb strength criterion; shear strength cohesion value decreases and then increases as a parabola with increase of the characteristics of soil particle size d10, d30, d50, d60, d90; friction angle values increase and then decrease as a parabola with increase of the characteristics of soil particle size d10, d30, d50, d60, d90; relations between cohesion and friction angle versus coefficient of uniformity and curvature coefficient are not obvious.