The reliability of submersible motor is the key to determining the stability of the middle and deep sea exploration and development equipment. In this paper, in view of different equivalent modes of convective heat transfer of housing surface, two temperature field models are established in the finite volume method (FVM). The unidirectional and bidirectional coupling mode between loss and temperature of motor are analyzed, and the temperature calculation results are compared when the oil friction loss and copper loss are constant and variable. Then, the bidirectional coupling mode between radiator temperature field and cooling system internal flow field is determined based on the structural parameters of radiator and cooling circulation impeller. The circulation flow is simulated and the influence of different loss unidirectional coupling modes on the temperature field is compared. Finally, the cooling system design procedure is proposed to obtain the matching relationship of the same cooling system structure parameters of the two temperature field models. Experimental validation is presented, and a more reasonable temperature field model is obtained under the coupling mode proposed. It provides the necessary theoretical basis for the research on the submersible motor cooling system.