Abstract: In order to know the strengthening mechanism of composite foundation with cast-in-situ concrete thin-wall pipe pile,static load tests were conducted in Yan-Tong Expressway.It was shown that the piles were of good quality and high bearing capacity.A 3D model was established for the composite foundation with cast-in-situ concrete thin-wall pipe piles.It was shown by the results of finite element analysis that the present model was reasonable.Based on the model,the effect of cushion stiffness,pile spacing,pile length and soil modulus on the bearing capacity was analyzed.It was indicated that cushion could well adjust pile-soil stress to make piles and soil work together.While the requirements for the bearing capacity of piles and soil could be well developed with large pile spacing,the bearing capacity and deformation should be met.The increasement of pile length could well exert bearing capacity of piles.The large soil modulus was beneficial to the development of shaft friction.