With the development of electric drive systems for electric vehicles toward high power density and integration, electric drive assemblies integrating the drive motor, motor controller, and reducer have attracted lots of concerns. Under the combined effect of multiple heat sources within a compact space, the system heat dissipation of an electric drive system remains an open challenge, so it is necessary to analyze the temperature rise characteristics inside the motor. This paper examines in detail the distribution of internal losses in the motor and calculates the thermal resistances of the motor components. Then, a thermal–hydraulic coupled simulation platform on the Amesim platform for predicting the temperature rise of oil-cooled motors is developed based on the lumped-parameter thermal network method. A bench test is established to verify the accuracy of the simulation model. Experimental results show that the maximum error is 5.53%. This study provides a reference for the thermal analysis of motors in electric drive assemblies.