Through macro performance test, chemical analysis and microscopic analysis, this paper studied the diffusion of sulfate ions and strength development of magnesium potassium phosphate cement (MKPC) specimens immersed in a Na₂SO₄ solution with a mass fraction of 5%. The results show that with the extension of soaking age, the content of sulfate ions inside the specimens and the depth of erosion gradually increases. The relationship between the depth of erosion of sulfate ions in MKPC specimens and the sulfate ion content generally fits a second-order or third-order polynomial (with a correlation coefficient R² greater than 0.998). By solving the analytical expression using Fick's second law of diffusion with established boundary conditions, the diffusion coefficient of sulfate ions in MKPC specimens is in the range of 10^-7 mm²/s, which is one order of magnitude lower than that of Portland cement concrete. The diffusion coefficient of sulfate ions in MKPC specimens exhibits a decreasing trend for soaking age not exceeding 180 d. After more than 180 d, the diffusion coefficient of sulfate ions in MKPC specimens gradually increases. After 360 d of soaking, the diffusion coefficient of sulfate ions in MKPC specimen is 3.9×10^-7 mm²/s, and the sulfate ion content at 2 mm from the surface is 0.218% (mass fraction). The strength of MKPC specimens increases initially and then decreases as the soaking age extends, following a similar trend to that of the diffusion coefficient of sulfate ions. After 360 d of soaking, the loss rates of flexural and compressive strength of MKPC specimens are both less than 5%.