Abstract: Magnesium potassium phosphate cement (MKPC) exhibits outstanding fire resistance in the field of steel structure refractories due to its high strength, high-temperature stability, and excellent bonding with metals. This study used forsterite as the matrix material and employed functionally modified diatomite loaded with sodium sulfate (Na₂SO₄/DE) to prepare a novel MKPC-based refractory coating. With the mass ratio of forsterite to phosphate (M/P) fixed at 1.85∶1, the effects of different Na₂SO₄/DE doping concentrations on the coating's mechanical properties, thermal stability, and fire resistance were investigated. Using SEM-EDS, TG-DTG, and XRD characterization techniques, the phase evolution and refractory mechanisms of the Na₂SO₄/DE-MKPC system under high temperatures were revealed. Experimental results indicate that at a Na₂SO₄/DE concentration of 20%, the coating achieves optimal comprehensive performance. Its bonding strength reaches 1.05 MPa, which is a 24.1% improvement over the unmodified system. After a 60-minute fire test, the backside temperature of the steel plate was reduced by approximately 192℃. The underlying mechanisms can be attributed to: (i) The scattering of heat flow by the pore channels of diatomite and the endothermic melting of Na₂SO₄; (ii) Forsterite acting as a high-temperature-resistant skeleton, enhancing the coating's fire resistance through its excellent thermal stability; (iii) The synergistic effect of Na₂SO₄/DE in strengthening the bonding with MKPC and suppressing high-temperature crack propagation. This research provides a theoretical basis for developing novel high-performance MKPC-based refractory materials.