Steel　slag　powder　was　used　as　a　mixed　material　of　magnesium　phosphate　cement　(MPC)　to　improve　its　mechanical　water　resistance.　The　mechanism　of　steel　slag　on　MPC　was　studied　through　the　composition　and　microstructure　test　of　MPC　hydration　products.　The　results　showed　that　the　strength　of　magnesium　phosphate　cement　gradually　decreased　with　the　increase　of　immersion　time　after　adding　steel　slag.　Adding　15%　steel　slag　could　improve　the　water　resistance　of　MPC,　make　the　structure　compact　and　reduce　the　strength　loss　rate.