Abstract: Traction motor is the power source of the railway locomotive, therefore, the stability and reliability of the locomotive transmission system are directly affected by the service lives of its key components (e.g., the support bearings, etc). For the heavy haul locomotive, the traditional evaluation methods are mainly based on the fixed load conditions, therefore, it is difficult to accurately evaluate the service lives of motor bearings under complex external excitations such as track random irregularities. Based on the vehicle-track coupling dynamics, a locomotive-track coupled dynamics model with traction power transmission system considering the specific structures of the traction motor and rolling bearing is established. In addition, the effect of the impact wheel-rail interaction and gear engagement is considered in this study. Besides, the service lives of traction motor bearings in the complex vibration environment of a locomotive are assessed based on the linear damage accumulation criterion and ISO 281 standard. Results indicate that the vertical wheel-rail interactive force, gear meshing force, the centrifugal force of rotor, and unbalanced magnetic pull increase obviously under the external excitation induced by the track random irregularity. In the complex vibration environment of the locomotive, the interactions between the roller and race in motor bearings are intensified, and the corresponding fatigue lives of the driving end and non-driving end motor bearings are shortened. Furthermore, with the continuous deterioration of the line state and the improvement of the locomotive running speed, the predicted life mileages of the motor bearings decrease in the increasingly harsh service environment. Because of the large external dynamic loads, the service life of the driving end bearing is shorter than that of the non-driving end one. This evaluation method proposed in this study can provide a theoretical guidance for the design, selection, and service life prediction of motor bearings used in the locomotive.