The strengthening effect of phosphorus slag powder (PSP) in cement-based materials was researched by model of strengthening effect of mineral admixes, and the mechanism of strengthening effect of PSP was also studied by SEM. The results indicate that, in early hydration period, the strengthening effect of PSP on strength of cement mortars is zero. At 90 days, strengthening effect has been exerted, and increases with the amount of PSP when that is no more than 40%, but decreases with larger content of PSP. At 180 days, the strengthening effect was further enhanced, the greater the amount of PSP, the greater its effect. For compressive strength of cement mortars, the strengthening effect of PSP is more than that of PSP on flexural strength. The results analyses of SEM show that, strengthening effect of PSP mainly come from its pozzolana reaction. With double excitation of cement hydration product hydroxide and gypsum, the low alkalinity C-S-H was produced.