The geometric model of ZM5 alloy casting was established by using the three-dimensional modelling software UG. The numerical simulation of ZM5 alloy casting under air-cooled, oil-cooled and water-cooled conditions was carried out by ProCAST software, and the casting was tested under air cooling conditions. The results show that the temperature distribution gradient of the casting is the most widely distributed in the water-cooled medium, followed by the oil-cooled medium, and the smallest in the air-cooled medium. Under the air cooling medium, the isolated liquid phase region of the casting is ellipsoidal; under the oil cooling and water cooling medium, the isolated liquid phase region of the casting is needle cone shaped. The test results are consistent with the simulation, which verifies the feasibility of the simulation. The simulation can provide a reference for the selection of different cooling media for magnesium alloys in the actual casting process.