Investigation was launched on hydration characteristics of cement based materials with phosphorus slag powder with employment of scanning electron microscope(SEM), X ray diffraction(XRD), differential scanning calorimetry thermogravimetry(DTA TG) and microcalorimeter. Results indicate that phosphorus slag powder only has influence on category and amount of hydration products, but no influence on type of hydration products, and there is no sign of hydroxyapatite. Phosphorus slag powder has no effect on hydration of C3A, but it will delay hydration of C2S and C3S in cement clinker. The setting retarding of cementitious system is mainly caused by the postponement of hydration induction period. Hydration hindrance is reduced after induction period while hydration speed is raised up when phosphorus slag powder is added. The general hydration degree of the composite paste remained lower than cement paste which is mitigated with development of age.