The surface flatness of molecular beam epitaxy Si-based HgCdTe material has an important influence on the preparation of infrared focal plane components.The Si-based composite substrate of molecular beam epitaxy as the substrate material of HgCdTe molecular beam epitaxy has a greater impact on the surface flatness of the final epitaxial material.In this paper,by studying the influence of various epitaxial factors on the surface flatness of Si wafers and substrate materials during the process,it is found through experiments that the high temperature effect of the deoxidation process is the largest,followed by the surface flatness of the Si wafer itself and thickness of the epitaxial film.Therefore,to obtain a Si-based composite substrate with low surface flatness,the following three conditions must be met:low Si wafer surface flatness,low desorption process temperature,and low epitaxial film thickness.