Propagation characteristics of crack cross different elastic material interface is important to hydrodynamic fracture of control technology .It makes the specimens of rock and similar materials including interfacial layer to do theItype crack through three-point bending experiments. The text shows that the crack occurred deflection through the interface. Based on damage mechanics, establish the plasticity damage model to describe the sand-mudstone damage behavior. Using the numerical solution technology to simulate the loading cracking process, the experimental and numerical simulation results are in good agreement. The main reason causing the fracture deflection is that the difference of sand-mudstone mechanics of materials property results in interface shear stress increasing, and result in the Itype crack expand transform to the I-IImixed type mode crack. TheII type shear crack makes the crack lateral spreading on the interface. Finally, leading to fracture deflection, the extent of deflection is associated with materials properties of mudstone, Shale content is more low, the deflection is more weak, and vice versa. It is important significance to controlling hydro-fracturing fracture extension.