Abstract: By means of energy method and minimum potential energy principle of elasticity, the elasticity theory analysis of infinitesimal deformation was performed for the concrete filled steel tube (CFST). The hooping mechanism coefficient, theoretic formulas of the composite axis pressed elastic modulus were gained. Analytical results indicate that the hoping coefficient is not only influenced by the Poisson’ ratio, ratio of diameter to wall thickness and the elastic modulus of the concrete, but also influenced by the Poisson’ ratio, diameter of section and the elastic modulus of the outer steel shell. The calculated results by the theoretic formulas are approximately equal to the results gained by unified theory. The theory formulas can actually and reliably describe the elasticity behaviors of infinitesimal deformation and the hooping mechanism of CFST.