The salt frost resistance of magnesium potassium phosphate cement(MKPC) added with steel slag powder or fly ash was investigated in the freeze thaw cycle test in fresh water and sodium sulfate solution by measuring the strength, apparent morphology, mass loss, linear expansion deformation, water absorption, phase composition and microstructure. The results show that both steel slag powder and fly ash can reduce the strength loss and mass loss of MKPC in salt freezing. The addition of appropriate amount of steel slag powder and fly ash can form good grain gradation with MgO powder, increase the density of MKPC hardened body. The existence of sulfate in freeze thaw can reduce the dissolution of calcium ions in slag powder. Fly ash can significantly improve the linear expansion deformation in freeze thaw cycles, and its inert SiO2 filling and morphological effects are beneficial to enhancing salt frost resistance of MKPC.