On the basis of discussing the damage mechanism of cushion cap, a method of optimum design for pile group foundation is put forward in this paper.The piles are decorated on the cushion cap which makes the area of cushion cap minimum.The object function of optimum design for cushion cap is its volume and the binding functions are the requirements of shear, column die cutting, corner-pile die cutting and settlement for cushion cap.The optimum design has been done by a computer program. This method is used to design a factory building. The final result suggests that this method is highly effective and practical. A fair-scale economic efficiency can be obtained in ifs practical use.