In order to solve the situation that the maximum temperature and maximum temperature difference of lithium battery pack may exceed the suitable temperature range when it works at 2.5 C discharge rate and 308.15 K ambient temperature. The heat dissipation model of the battery pack based on composite phase change material (CPCM)/liquid-cooled composite is established. Firstly, the related performance parameters of the lithium battery were measured by experiments. Then, the influence of CPCM thickness on the heat dissipation performance of battery pack was discussed by numerical simulation method. The analysis shows that the maximum temperature of the battery pack could not be controlled within an appropriate range by a single phase change material cooling mode when the thickness of CPCM varied within a certain range. Therefore, CPCM/ liquid-cooled composite heat dissipation method was proposed, and the Multi-objective optimization design was carried out with the thickness of composite phase change material, liquid-cooled channel spacing and liquid flow rate as the design variables, and the maximum temperature and maximum temperature difference of the battery pack as the optimization objectives. The results show that, the maximum temperature and maximum temperature difference of the optimized battery pack are 316.88 K and 0.30 K respectively, which meet the design requirements, but there is a risk of leakage of the phase change material during the phase change process. Compared with the single phase change material cooling method, The optimized composite cooling model can significantly reduce the maximum temperature of the battery pack, while controlling the maximum temperature difference within a safe range. While ensuring high thermal conductivity of the outermost packaging structure of the heat dissipation model, the structural design should be strengthened to prevent the leakage of phase change materials.