Blast-furnace slag was used as an active addition of magnesium phosphate cement. The influence of blast-furnace slag on the setting time, mechanical property, mineral phase and microstucture of slag blended magnesium phosphate cement (MPC) was studied. The prescription was designed as follows: the ratio of monoammonium phosphate to magnesia was 25% in mass, the borax additive amount to magnesia was 7.5%, the additive amount of blast-furnace slag was 0%, 10%, 20%, 30% and 40% separately in the ratio of total amount of dead burned magnesia and monoammonium phosphate to MPC. The results show that slag might participate the hydration, which improves the bonding of MPC, and the compressive strength of slag-blended MPC is improved with the increasing additive amount of slag. However, slag might results in expansion stress during hydration process, which causes lattice disturbance in hardened MPC, so the flexual strength decreases with the increasing additive amount of slag at the same time. The main hydration product of slag blended MPC is struvite (MgNH₄PO₄·6 H₂O), the content of amorphous phase might increase after being mixed with slag in MPC, and Ca²⁺ ions in slag can result in solid solution in the crystal lattice of struvite, which changes the microstructure and size of the hydration product. There is also much magnesia and slag powders subjected to no reaction in the hardened cement, which can be used as aggregates.