The air-cooled universal motor used in the household high pressure washer has a problem of overheating in the stator winding. A three-dimensional fluid-solid coupling heat transfer model of the motor is built using an example of an HC8840F air-cooled single-phase series motor. On this foundation, the internal flow field and the temperature field of the series motor are simulated using the fluid-solid coupling heat transfer calculation method. To confirm the validity of the three-dimensional fluid-solid coupling heat transfer model of the universal motor, the stator winding of the motor is subjected to temperature rise test under various working conditions. On this basis, the guidelines for the design of structural optimization are provided. From the perspective of consumption reduction design, the impact of rear fan on the internal flow field and the temperature field of the motor is analysed. The findings demonstrate that the internal fan alters the airflow and make the average temperature of the stator winding decrease by 3.3 ℃.