Background: Because the permanent magnet servo motor (PMSM) has the advantages of high efficiency, big torque, energy saving and environment protection, more and more researchers pay attention to it in recent years. This paper did research on the application of the equivalent thermal network method in the motor temperature field calculation.Methods: The basic theory and the calculation of traditional thermal network method were analyzed. Considering the relatively high deviation of traditional thermal network method, the paper proposed a new method of decreasing the deviation. Taking a 10kW PMSM as an example, the theories of energy conservation and heat conservation are considered, and a new thermal network node model was built. In this model, the key node division was optimized in the position where the heat transfer difference is great. The difference of heat transfer capacity at the edge of the thermally conductive materials and the influence of external heat flow on the calculation of equivalent thermal resistance had been considered properly. Results: The temperature field of the motor was calculated by traditional and optimized thermal network methods respectively. In order to verify the optimized method could improve the accuracy of the traditional method, the temperature field of the motor was also calculated by the finite element method. Comparing the results of traditional and optimized methods with the finite element method, it proved that the optimized method could decrease the difference between the results of equivalent thermal network method and finite element method, and the practicability of the optimized method is also proven. Conclusion: The optimized method provides a new method to increase the accuracy of equivalent thermal network method.