Taken a 315 kW permanent magnet synchronous motor (PMSM) with low speed and high torque as an example, it is analyzed in this article that the different water inlet velocity has important influence on the heat dissipation at the same waterway structure. The relationship between the velocity and temperature rise of cooling water inlet is summarized, and an optimal velocity is obtained. The difference between traditional fluid solid thermal coupling simulation and magnetic thermal bidirectional coupling simulation is compared, and the former has a faster speed. Although the latter has a slower speed, it can well show the influence of material temperature property on the temperature field. This paper provides diversified reference opinions for the simulation methods used in different stages of temperature field analysis.