Abstract: In order to study the effect of temperature variation on thermal stress and thermal fatigue life of cast aluminum alloy casing of a vehicle permanent magnet synchronous motor under variable working conditions, this paper combines theories, experiments and finite element simulation to conduct a multi-physical field coupling study on the casing. Firstly, the ambient temperature of the motor as the boundary condition for simulation calculation, and the winding end temperature as the reference of the simulation results are collected experimentally. Then, the main losses of the motor are calculated, and the temperature field distribution of the motor casing is obtained through simplified modeling and finite element simulation of the motor based on heat transfer and fluid dynamics. Afterwards, basic material parameters of the thermal fatigue life prediction model of the cast aluminum alloy are obtained through the experiment. The thermal stresses of the casing under different temperature distributions are also calculated by combining the temperature field results. Finally, the thermal fatigue life of the casing under the thermal stresses is obtained as 29 673 times. This study provides a certain reference basis for the design optimization of the motor casing and the selection of materials.