Superficial phenomenon and mass loss of magnesium phosphate cement(MPC) in aqueous alkali were studied. The corrosion products at 3 and 28 days were detected by XRD and SEM/EDS. The experimental results show that alkaline resistance of MPC is weak, while its alkaline resistance is improved by incorporated fly ash. The exfoliation of MPC and its loose structure exhibite weak alkaline resistance of MPC, which is caused by chemical reaction（product：magnesium hydroxide） between potassium magnesium phosphate hexahydrate colloid and aqueous alkali.