Experiments with expanding agent proportion（by mass） of 4%, 8% and 12% were carried out respectively in order to clarify the effects of expanding agent proportion and steel ratio on concrete filled steel tube(CFST)s creep characteristics. The results show that 4% of expansive agent additive affects the creep degree most, the 8% of expanding agent additive does less, and the 12% of expansive agent additive does the least. When addition of expanding agent was decreased from 12% to 8%, the creep degree of concrete filled steel tube was slightly decreased. By means of SEM and EDS the action mechanism of expanding agent was clarified. The analysis revealed that expanding agent reacts with concrete to produce ettringite. On the one hand it may compensate shrinkage by slight expansion. On the other hand it may fill in pores and made dense for hardened cement. The concrete filled steel tube with tube wall thickness of 38%, 41%, 66%, 92% and 108% were prepared. It is shown that the exterior loads are shared with the increase of steel ratio. The creep degree is decreased. The steel ratio affects CFST creep mechanism on the same loading condition. The study concludes that the stress in concrete decreases along with increasing steel ratio. With the increase of wall thickness, steel would be binding the concrete and limiting the deformation of concrete.