Influences of ratio of acid to base on the properties of magnesium and potassium phosphate cement（MKPC） with composite retarder were investigated through testing hydration temperature of MKPC pastes, the compressive strength and water stability of hardened MKPC pastes and analyzing the phase compositions and microstructure of hardened MKPC pastes. The results indicate that there is an optimum ratio of acid to base when hardened MKPC pastes with composite retarder have high compressive strength in natural curing condition and high residual compressive strength ratio in water curing condition. Because of the effective delaying effect of composite retarder on the early hydration rate of MKPC paste, the optimum proportion of acid component of MKPC with composite retarder is decreased above 50% compared to the MKPC paste with borate retarders，which reduces its cost above 25%. In hardened MKPC paste with matched ratio of acid to base, more hydrates are formed and grow perfectly and stably with few defects and cracks insulting in perfect microstructure.