10 concrete filled steel tube（CFST） specimens with different strength grades and sizes were cured under pressure by self-designed hydraulic device. The effects of curing pressure and its duration on uniaxial compressive strength， stress-strain variation and failure characteristic of CFST specimens were studied. Microscopic test methods were used to study the development and formation of pore structure and hydration products. The results show that the particles in the mixture are squeezed close to each other under pressure curing， and the formation and development of microcracks are inhibited， which lead to the higher compactness of core concrete. Moreover， the crystallization process of hydration products are promoted， and the pore structure can be fully filled by the hydration products， which improves the strength of core concrete. In addition， the pressure causes the circumferential tensile strain of steel tube， which provides pre-compressive stress on core concrete after final setting time. Therefore， the uniaxial compressive properties of the CFST are improved.