By means of SEM, XRD and calorimeter C 80Ⅱ, the hydration characteristics and hydration kinetics of cement based materials with and without P2O5 incorporated were investigated. It is found that the amount and size of hydration products incorporated with P2O5 were decreased dramatically. When 3.5% P2O5 was added in cement, total amount of hydration heat was decreased by 32.6% and initial and final setting were delayed by 1.10 h and 12.54 h respectively. In terms of mechanism analysis, P2O5 had little impact on cement hydration mechanism, and accelerating stage was controlled by chemical reaction, decreasing stage controlled by both chemical and diffusion reaction while stabilizing stage controlled by diffusion alone. The hydration resistance in both accelerating and decreasing stage was elevated while decreased in stabilizing stage. The activation energy in both accelerating and decreasing stage were reduced and increased in stabilizing stage. P2O5 was favorable to hydration of C3A yet retarded hydration of C3S and C2S.