The breakdown strength of polypropylene (PP) for film capacitors drops sharply in a high-temperature environment, which would lead to capacitor insulation failure frequently. This paper proposes a method to improve the breakdown strength of PP at high temperatures by introducing long-chain branches (LCBs). Different from traditional linear PP, the results show that LCBs can increase the number of spherulites and refine the grains due to heterogeneous nucleation, resulting in a reduction in weak areas in the films and an increase in trap densities and energy levels in long-chain branched polypropylene (LCBPP). In addition, the higher melt strength of LCBPP indicates that LCBs promote entanglement between molecular chains and hinder the sliding of chains, thereby reducing the influence of temperature on the free volume. Therefore, compared with PP, the conductivity of LCBPP is lower at high temperatures, and the breakdown strength increases by 16.2% at 105°C. This method provides an idea for improving the breakdown strength of PP films at elevated temperatures from the perspective of regulating the microstructure.