In order to better promote the engineering application of magnesium oxychloride cement (MOC), the effects of raw material ratio, single mixing of different phosphate modifiers, combined mixing of phosphoric acid and mineral admixture modifiers on the water resistance of MOC were studied. The improvement effect of modifier on the water resistance of MOC was comprehensively evaluated by using flexural strength, compressive strength (mechanical properties), variation coefficient (strength stability with modifier dosage) and water resistance coefficient (water resistance), and the improvement mechanism of water resistance was analyzed by scanning electron microscope. The results show that when the molar ratio of raw materials is n(MgO)∶n(MgCl2)∶n(H2O)=7∶1∶15, and 1.0% phosphoric acid and 60% silica fume (mass fraction of MgO), the water resistance of MOC is the best, and the water resistance coefficient of modified MOC is above 1.1. Microaggregate filling effect, pozzolanic effect and enhancement effect of strength phase (P5) stability together improve the water resistance of MOC.