With the development of electronic technology, the power density of permanent magnet synchronous motors (PMSM), which are used in the ships, is increasingly significant. In order to investigate the characteristics of thermal distribution and the temperature variations caused by the structure and materials of main components, a 50 kW surface PMSM, used in warships, was researched. Based on the computational fluid dynamics (CFD) and the numerical heat transfer theory, a 2D physical model was established according to the conjugate heat transfer theory. Therefore, the start-up and steady-state temperature rises of PMSM, operating under frequency conversion control, were simulated numerically using the finite volume method. The correctness of calculated results and the accuracy of the solution method were verified by experimental data. What's more, the changes of factors, such as the height of cooling fins, the materials of motor casing, and the air flow rate, which may influence global temperature, were studied. Some useful conclusions are obtained, which has practical engineering value and reference value to the design of PMSM.