Abstract: The reasonable design and accurate analysis of the cooling system is the key to the stable operation of underwater motor. In this paper, firstly, considering the problems of pressure balance, cooling and sealing, an oil-filled underwater induction motor cooling system was designed. Secondly, the unidirectional and bidirectional coupling relationship between loss and temperature in the underwater motor were analyzed, and the corresponding calculation procedure was given. Based on the mutual influence of radiator, underwater motor and circulating impeller on the temperature and pressure loss, bidirectional coupling relationship between temperature field and flow field in the cooling system was determined, and the equation for computing radiator performance parameters was derived. On this basis, the coupling analysis method of multi-physical fields was proposed. Finally, the proposed method was used to analyze the cooling system of 55 kW underwater motor. Variation laws of copper loss, oil friction loss, circulation flow rate at operating point and radiator outlet temperature are revealed. It is pointed out that it is necessary to calculate rotor iron loss in the analysis of underwater motor temperature field. The influential laws of radiator structure parameters and their reasonable matching range are obtained. The rationality of this method is verified by experiments.