This paper investigated the magnesium sulfate resistance of mortar incorporating coral sand powder (CSP) combined with supplementary cementitious materials (SCMs). With erosion in magnesium sulfate (MgSO4) solution at 5 and 20 ℃ for 365 d, the magnesium sulfate attack resistance of mortar was analyzed through appearance, length change, compressive strength, X-ray diffraction pattern and Fourier transform infrared spectroscopy. It is found that the mortar with CSP deteriorates seriously when it is eroded to 365 d in magnesium sulfate solution at 5 ℃. Sulfate resistantance Portland cement (SRPC) with 20% (mass fraction) CSP exhibits better magnesium sulfate attack resistance than ordinary Portland cement (OPC) with 20% (mass fraction) CSP. Fly ash and slag can effectively improve the magnesium sulfate attack resistance of mortar with CSP at 5 ℃, while silica fume reduces the magnesium sulfate attack resistance at 20 ℃. At 5 ℃, the mortar with CSP mainly undergos magnesium and thaumasite sulfate corrosion accompanied by a small amount of gypsum generation, while at 20 ℃, magnesium and gypsum-type sulfate corrosion mainly occurs.