Traditional submersible motors have low slot full rate and large vibration, which increase the risk of damage to the winding insulation. A structure of combined stator module submersible motor is proposed, which can effectively increase the slot full rate to reduce motor vibration. However, under the working conditions of high temperature and high pressure, the temperature rise of this special structure motor needs to be further designed and checked. Due to difficulties in motor temperature test under actual operating conditions, to ensure the stability of the motor running, temperature rise of ultra-slim long submersible permanent magnet motor is studied. Its calorific value can be approximated by thermal load and verified by finite element method. The temperature rise distribution of the motor under extreme conditions is calculated, which provides an important basis for the design of ultra-slim long modular combined submersible permanent magnet motor (MCSPMM).