Abstract: The small underwater device plays an important role in the field of ocean resource detection and utilization. With development of underwater device towards multi-function, intelligence, long endurance and high submergence depth, there are more and more power components in the device and their heat dissipation issue is increasingly serious. In this paper a thermal control module for a small underwater device is proposed based on phase change material (PCM). Passive heat absorption with PCM is adopted for heat dissipation of the power components in the device. The thermal control performance of the PCM module under typical thermal power conditions is studied and different types of PCM are compared. The results show that low melting point metal PCM gallium can significantly suppress the temperature rise of the heat source and extend the normal working time of the device to 2.7 times as compared with original natural cooling method. However, when organic PCM octadecane is used, the heat dissipation performance is deteriorated, mainly due to the poor heat transfer performance of organic PCM, which hinders the heat transfer through the hull. The PCM thermal control module proposed in this paper has advantages of zero power consumption, high heat storage density and stable working performance. Filling the original hull space with PCM does not increase the volume of device and does not change the internal space of device. Therefore, natural cooling and heat absorption with PCM can be adopted for heat dissipation simultaneously. This can effectively prevent overheating damage of internal electronic components and ensure continuous, safe, stable and efficient operation of the underwater device.