The high-temperature resistance performance of film capacitors in motor controllers faces greater challenges with the development of high voltage and high power density of new energy vehicles (NEVs). In this study, a typical film capacitor is taken as the object of research; simulations of temperature fields and experiments on temperature rises are conducted. Additionally, a heat dissipation structure is designed to enhance the high-temperature resistance performance of the capacitor. Firstly, the temperature field distributions of the capacitor under different ripple currents and ambient temperatures are analyzed using ANSYS electro-thermal coupling simulation. Then, under the same operating conditions, experiments on the temperature rises are conducted on the capacitor, which is installed with K-type thermocouples to verify the accuracy of the electro-thermal coupling simulation. Furthermore, an external heat dissipation structure consisting of a micro-channel liquid cold plate is designed for the capacitor, and the effectiveness of the external cold plate is verified through electro-thermal coupling and FLUENT simulations. It is found that under the operating conditions of 85 °C ambient temperature and 175 A ripple current, the maximum temperature of the capacitor with the external cold plates decreases by 46.08% compared to the original capacitor. The results indicate that the external cold plates effectively improve the high-temperature resistance capability of the film capacitors in electric vehicles.