Abstract: The ice-liquid curve in phase diagram of the antifreezer solution and its formula are deduced from measured value of freezing point. A new concept called "liquid water-cement ratio" and its formula are advanced, which can be used to evaluate liquid water amount in the concrete mixed with antifreezer at negative temperature. Based on this theory, mechanism of antifreezer is explained as follow:at a pointed negative temperature, the concentration of antifreezer in liquid keeps constant, the liquid water-cement ratio in concrete is only determined by the amount of remainder of antifreezer; therefore, the liquid water consumed in cement hydration is compensated by thawed ice and the liquid water will never disappear, the cement hydration will continue on. the optimum values of liquid water-cement ratio and the antifreezer dosage exist in the concrete mixed with some antifreezer at negative temperature.