Abstract: In tins paper,the forming process of aluminium-alloy casting surface has been studied using investment shell mold and the law on roughness formation of the casting surface is found out.Experimental results show-ed:fine protrusins are sensitive to the casting surfaces;fine holes pre-sented on the mold wall are not sensitive to the casting surfaces;all casting surfaces consist of both copy surfaces and free surfaces.If the mold wall is rather smooth and compact,the surface finish of the cast-ings could only be improved by improving the capacity of the alloy copy surface and increasing its proportion.