CFG pile composite foundation is used to deal with weak foundation，it can increase the bearing capacity, and reduce the foundation settlement. In the process of designing CFG pile composite foundation，thickness of cushion is very important. It is one of most important factors in the bearing capacity of CFG pile composite foundation.But there is no theory method in designing the thickness of cushion. This article, based on the theory research, elicits the theory method of rational thickness of cushion. It offers expressions about the rational thickness of cushion and pile-soil stress ratio.