The　poor　water　resistance　of　traditional　magnesium　phosphate　cement　(MPC)　limits　its　wider　application　in　engineering　fields.　So　far,　research　on　enhancing　the　water　resistance　of　MPC　has　been　relatively　limited.　Therefore,　this　paper　optimizes　the　slag　content　based　on　magnesium　silicon　potassium　phosphate　cement　to　prepare　slag-magnesium　silicon　potassium　phosphate　cement　(S-MSPPC).　The　research　results　indicate　that　slag　contains　various　active　components,　and　its　appropriate　addition　can　refine　the　pore　structure　of　the　paste.　With　the　increase　in　slag　replacement　rate,　the　paste　setting　time　increases,　and　the　fluidity　decreases　slightly.　The　hydration　of　silica　fume　to　produce　SiO(OH)3-　and　SiO2(OH)22-　ions　significantly　enhances　the　alkalinity　of　the　paste　and　provides　conditions　for　activating　the　activity　of　the　slag.　The　S-MSPPC　hardened　paste　can　maintain　alkalinity　for　the　long　term.　Moreover,　the　water　resistance　of　the　S-MSPPC　paste　was　significantly　improved　while　ensuring　mechanical　properties.　The　water　resistance　coefficients　of　S-20　with　20　%　slag　increased　by　49.23　%　and　40.64　%　at　28　days　and　56　days　compared　to　the　reference　group　S-0.　This　improvement　was　due　to　the　participation　of　silica　fume　and　slag　in　the　hydration　reaction　to　form　M-S-H　and　C-(A)-S-H　gels　in　the　paste.　These　gel　products　fill　the　pores　and　microcracks　in　the　paste,　optimize　the　pore　size　distribution,　reduce　porosity,　and　enhance　density.　In　addition,　the　alkaline　environment　within　the　paste　effectively　slows　the　dissolution　rate　of　K-struvite　in　water,　further　enhancing　the　water　resistance　of　S-MSPPC.　©　2024　Elsevier　Ltd