The compound protecting structure of foundation pit, which combines post reinforcing jet grout piles with bored piles, is a new type of structure designed to meet the need of the special environment of an actual project. It has the advantages of high anti bending ability, good leak proof, lower cost and easier handling in building structure. It can be widely used in other cases. The mechanical model of calculating the bending resist ability of the cross section of the structure and the design method of the structure are presented in this paper.