The invention discloses a liquid curing agent for a phosphate casting binder and a preparation method thereof. The liquid curing agent comprises 60-63% of metallurgy magnesia powder, 3.16-4.36% of sodium bentonite, 1.15-1.88% of polyvinyl butyral, 0.45-0.65% of silica sol, 0.35-0.54% of surfactant and the balance alcohol. The preparation method comprises the following steps of 1, adding the metallurgical magnesia powder, the polyvinyl butyral and the sodium bentonite into a mixer, mixing and stirring in a dryness state to obtain a mixed powder; 2, diluting the silica sol and the surfactant inhalf of the alcohol to obtain a mixed solution; 3, slowly adding the mixed solution to the mixed powder, stirring while adding, sealing the mixture after the addition of the mixed solution is completed, and kneading the mixture for 1 hour to make the mixture be a dry paste; 4, adding the remaining alcohol to the mixture to dilute the paste to obtain the liquid curing agent. The liquid curing agentprepared by the method has a suspension rate of 98% in 48 hours, has very stable Baume degrees, does not precipitate and does not become thick during long-term storage, and has high stability.