The permanent magnet brushless motor for deep-sea propulsion operates in the low-temperature and high-pressure sea, and the motor is filled with oil. Its heat sources and heat dissipation conditions are different from ordinary industrial motors. Accurate calculation of temperature rise of the propulsion motor has guiding significance for its high power density and lightweight design. Taking the main propulsion motor used on “Siyuan” full-ocean-depth autonomous remotely-operated vehicle (ARV) as an example, a three-dimensional steady-state flow field-temperature field simulation model including the ducted propeller, propulsion motor, internal oil and external water is built based on computational fluid dynamics and heat transfer theories. The influence of oil filling and propeller rotation on the motor temperature rise is studied, and the temperature distribution of its main components is analyzed. Finally, the effectiveness and accuracy of the simulation analysis are verified by the temperature rise experiment.