To study the mechanism and failure mode of the joint force of steel casing and reinforced concrete（RCFST）,this paper takes the phase II pile foundation engineering of Chongqing Orchard Port as the prototype to carry out the research on the cooperative force mechanism of steel structure and reinforced concrete structure and its design optimization technology.The axial pressure of the short column of RCFST structure is carried out with the thickness of the steel tube,the ratio of reinforcement and the ratio of the stirrup.According to the test results of different parameters,the failure mode and stress mechanism of RCFST structure are analyzed.The test results show that the steel retaining tube and the reinforced concrete short column structure have the restraining effect on the core concrete,and the double restraining effect is obvious.The restraining effect of the steel retaining tube and the stirrup on the core concrete can effectively restrain the concrete crack development and make the steel retaining tube more traditional steel tube concrete structure.It has higher bearing capacity and better plasticity & toughness.When the steel retaining tube has sufficient restraining effect,the steel tube,concrete and longitudinal reinforcement can be yielded when the specimen is destroyed,and the material properties are fully utilized.