Underwater multi-module constant-current-to-constant-voltage converters, which convert the high-voltage constant current from the backbone cables to low-voltage constant voltage for supplying each observation instrument, have been widely used in submarine observation networks. However, the submarine environment is harsh and the device destabilization rate is high. Therefore, this paper proposes a finite element-based electro-thermal modeling approach to address the thermal management challenges of underwater multi-module constant-current-voltage converters in the submarine environment. Firstly, the topology and control strategy of the converter are introduced, the power loss model is built and its power loss is obtained. Then the finite element software is used to build a thermal model to simulate the temperature distribution under actual working conditions, and the accuracy of the model is verified through experiments. The results show that the established electric-thermal model can accurately describe the temperature rise of the converter, which provides a reference for the life prediction of the converter and the reliability analysis of the power electronic system.