The strength and water absorption changes of potassium magnesium phosphate cement (MKPC) coating slurry soaked in salt solution were tested, and the phase composition and microstructure changes were analyzed. The following results were obtained: the residual flexural and compressive strength rates (89.5%, 115.7% and 83.7%, 98.1%) of reference specimens (M0) and modified specimens (M2) soaked in 3.5% NaCl solution for 540 days were significantly higher than those of M0 and M2 soaked in water for 540 days (75.0%, 95.5%, 74.3%, 91.6%). The initial water absorption rates of M2 specimen in water and chloride solution (0.66% and 0.71%) are lower than those of M0 specimen (0.74% and 0.72%). When immersed in water and chloride solution for 540 days, the water absorption rate of M2 specimens increased significantly (51.5%, 16.9%) compared to M0 specimens under the same conditions (73.0%, 30.6%).