To improve the thermal uniformity of liquid cooling system for the battery thermal management, a three-dimensional CFD model for the coupled heat and mass transfer in the liquid cooling system was established, which was consisted of a grooved aluminum vapor chamber and a one-through flow liquid cooling plate. The numerical model was solved by VOF method and verified by experimental data. The results show that the thermal uniformity can be improved by the vapor chamber. The temperature difference of the heating surface can be kept within 2.72 K, which is related to the heat and mass transfer process of working medium in the vapor chamber. With the impact of the temperature rise of the coolant, there is a directional flow of vapor phase in the longitudinal direction inside the vapor chamber. The heat energy of the vapor phase can be transported from the high-temperature region to the low-temperature region. Therefore, the influence of cooling water with large temperature rise on the temperature distribution of surface can be suppressed, and the thermal uniformity can be improved.