The traditional liquid cooling system of containerized battery energy storage power stations does not effectively utilize natural cold sources and has the risk of leakage. To address the above problems, a novel two-phase liquid cooling system with three operating modes was developed. An annual field test was carried out for containerized battery energy storage power stations. The analysis of thermal management effectiveness and energy conservation impacts during ongoing charging and discharging processes involves investigating the key factors that influence battery temperature and the characteristics of the cooling system, including uniformity of temperature, supply liquid temperature, and yearly energy consumption. The results showed that the cooling system can control the maximum temperature difference of 4 ℃ during continuous charging and discharging in different weather conditions, and the maximum EER annually reaches 10.3. During the spring transition season at 20 ℃, the system can still be cycled through natural loop cooling, with the fan continually running to maintain the supply liquid temperature at about 24 ℃. These results show that this novel system can effectively make full use of the natural cold source for energy-saving and can maintain temperature uniformity even in continuous charging and discharging conditions and high-temperature weather for containerized battery energy storage power stations.