Abstract: The hollow steel tubes of concrete filled steel tubular(CFST) structure are suffered vertical construction loads during construction as they are vertical support frame.The steel tubes will suffer initial stress before they can work together with concrete infilled.On the other hand,they are also subjected to horizontal hydrostatic loads of inner wet concrete as well as axial loads during concrete pouring because they play the formwork.The design of CFST that should be divided into two stages,and the design method and mechanical analysis at its construction stage are explored.It is possible and available way to solve the problem by FEM.The research method of civil engineering construction mechanics is also discussed.