In these years, the advantages of using phase change material (PCM) in the thermal management of electric power battery has been wide spread. Because of the thermal conductivity of most phase change material (eg.wax) is low, many researchers choose to add high conductivity materials (such as black lead). However, the solid-liquid change material has large mass, poor flow-ability and corrosively. Therefore, it still stays on experiential stage. In this paper, the Thermal characteristics of power battery firstly be invested and the requirements of thermal management system also be discussed. Then a new PCM thermal management has been designed which uses pure water as liquid phase change material, adopts PCM with a reflux device for thermal management. Meanwhile, utilizing the characteristics of pure water which are economical, easy to prepare, light weight, strong liquidity and hardly corrode the battery to explore the feasibility for water as a PCM for battery thermal management system. It is expected that the battery temperature could be kept around 45°C and the maximum temperature is not more than 50°C in this thermal management system. By the way, the difference of battery wall temperature and PCM should lower than 5°C.