Abstract: In order to study the effects of expansive agent dosage and lateral confinement by steel tube on creep strain of concrete, 12 groups with or without lateral confinement of steel tube were conducted. The test results show that creep strain of concrete in lateral confinement by steel tube is 0.175-0.181 times of without lateral confinement at loading age to 480 days, and lateral confinement greatly reduces the creep strain of concrete. Three creep strains of concrete were tested under mass fraction of 4%, 8% and 12% expansive agent dosage. The test results show that the expansive agent dosage is the same rule of creep strain effects on concrete in both lateral confinement and without lateral confinement of the concrete structure. When admixture is 4%, the creep strain of the structure is the biggest, and when admixture is 8%, creep strain of the structure take the second place. When admixture is 12%, the creep strain of the structure is minimal. Stress ratio of concrete structure by steel tube lateral confinement is less than that without lateral confinement under the same load stress. Lateral restraint of concrete is increased by steel tube confinement, and the longitudinal deformation of concrete is confined. The smaller the stress ratio, the smaller structure creep deformation. At the same time, the mechanism of the expansion agent was analyzed by scanning electron microscopy secondary electron imaging. The final analysis conclusion is that expansive agent in cement hydration produces three sulfur type calcium sulphoaluminate(AFt) crystals which is thick stick and cross each other. The AFt crystals fill in pores that it was occupied by a solution, which makes cement denser.