With the rigorous requirements of high power density for propulsion system, an oil-filled PMSM used for deepsea electromagnetic propeller was investigated in this paper. The heat sources in motor, including copper loss, stator iron loss and viscous power loss, were estimated, and especially the influence of temperature on power losses was also discussed in experiment. And then the heat transfer coefficients including the surface of the shell, and the gap filled with oil were proposed on the basis of analyzing of flow field. Finally, a 3D FEM thermal model was set up to compute the temperature distribution.