The unsteady working environment of water jacket was established by simulating the engine working state during the process of downhill and uphill. The flow and temperature fields of cooling water were acquired by analyzing the water flow and heat transfer under unsteady conditions with CFX software. The heat distributions of engine cylinder and cylinder head were further determined according to the fluidsolid coupling analysis. The test verification of variable conditions was finally conducted. The results show that the simulation method is feasible. It is also proved that the cold and thermal shock test can provide a necessary environment for the heat transfer analysis of water jacket.