Abstract: The present disclosure discloses a novel magnesium phosphate-alkali activated composite cementitious material with rapid hardening, early strength, and high water resistance and a preparation method thereof. The composite cementitious material is a mixture system of a magnesium phosphate cementitious material interweaving and coexisting with an alkali-activated cementitious material, where the alkali-activated cementitious material is prepared by alkali activation of an activatable mineral using a hydration product of a high-alkalinity magnesium phosphate cementitious material prepared from an alkaline hydrophosphate. The composite cementitious material obtained ensures excellent mechanical properties while actively converting part of or all of air-hardening material components into a hydraulic material, so that the problem of poor water resistance of the magnesium phosphate cementitious material can be effectively solved.