Polymer‐based dielectrics are attracting increasing attention due to their high‐density energy storage. However, mitigating the heat generation in real capacitors has been a challenge. Here an electrothermal breakdown phase‐field model is developed to fundamentally understand the thermal effects on the dielectric breakdown of polymer‐based dielectrics in real capacitor configurations including the increase in the dielectric loss and the decrease in the breakdown strength. While both enhancing the thermal conductivity and reducing the electrical conductivity of the polymer nanocomposites can reduce the thermal effects, it is found that reducing the electrical conductivity is more effective. This work is expected to not only stimulate attention to the thermal effects in polymer‐based dielectrics but also provide a fundamental guidance to mitigate the heat‐induced deterioration of breakdown strength. An electrothermal breakdown phase‐field model is developed to fundamentally understand the thermal effects on the breakdown behavior of polymer‐based dielectrics with different microstructures. The predicted temperature‐dependent breakdown strengths agree with the available experimental measurements. It is found that reducing the electrical conductivity is more effective than enhancing the thermal conductivity of the polymer nanocomposite to reduce the thermal effects.