In this paper, a multi-material composite casting model with variable wall thickness structure is builded. Firstly, by simulating the filling and solidification process of A356 alloy in the multi-material composite casting mold, the filling time and temperature field results of the multi-material composite casting mold were obtained. The solidification time of zircon sand with silica sand, chromite sand with silica sand decreased progressively. The solidification time of casting is shorter at the Connection position of zircon sand with silica sand and chromite sand with silica sand, and the solidification speed of metal liquid is faster. Secondly, analysis the Electron Backscatter Diffraction Analysis (EBSD), electron microprobe analysis (EPMA), tensile strength and scanning fracture of A356 alloy casting with silica sand, chromite sand and zircon sand composite casting in gravity casting. The results show that at the same wall thickness, the grain size of chromite sand and zircon sand castings is better, the concentration ratio of Al, Mg, Si and other elements is lower, the mechanical properties are increased, and the fracture is characterized by ductile fracture. At the same time, with the decrease of wall thickness, the grain size of sand castings of the same material is fine, the concentration ratio of elements is reduce, the mechanical properties are increase, and the fracture surface presents the characteristics of ductile fracture.