Taking quartz powder as a reference, the effects of fly ash and ground granulated blast furnace slag powder (GGBS) on the hydration process of magnesium phosphate cement were researched by monitoring the changes of hydration heat, suspension pH value and setting time. In addition, the compressive strength of mortar and pore structure of hardened paste were determined. The results show that fly ash as well as GGBS have strong delaying effects on the hydration of magnesium phosphate cement, and the delaying effect of GGBS is stronger. Fly ash and GGBS can improve the pore structure of hardened paste, and the improving effect of GGBS is more significant. When the replacement ratio is 15% (mass fraction), the compressive strength of magnesium phosphate cement containing fly ash or GGBS is higher than that of pure magnesium phosphate cement at 2 h. The compressive strength of magnesium phosphate cement containing fly ash is equal to or higher than that of pure magnesium phosphate cement at late ages. While the compressive strength of magnesium phosphate cement containing GGBS is significantly higher than that of pure magnesium phosphate cement at late ages.