Abstract: Foundation treatment is necessary when a structure is built over the soft clay ground.A composite foudation is one of effective reinforcement methods.In general,a project requires a higher bearing capacity in shallow soil and sufficient strength in deep soil to support the overlying soil layer.A long-short-pile composite foundation with a high replacement ratio in the shallow layer and lower one in the deep layer,satisfies demands for bearing capacity in soft foundation at diffenrent depths.Meanwhile,shallow composite modulus is high because of high replacement ratio,while deep composite modulus is low.As a result,high stresses will occur in the shallow soil and low stresses in deep one,which is a favorable distribution for decreacing total settlement of the soft foundation.A optimization mathematical model is developed and solved using complex method.A case study is given to demonstrate the optimization design method.The results show that the most reliable and economical design can be easily obtained from the presented method.