Abstract: High-power microwave technology shows great promise for applications in low-altitude security systems such as countering non-cooperative unmanned aerial vehicles. However, conventional high-power photoconductive semiconductor switches (PCSSs) based on impurity absorption intrinsically fail to achieve high voltage conversion efficiency across different operating voltages, limiting their overall performance. Here, we propose a novel PCSS based on phonon-assisted absorption, which fundamentally breaks the efficiency bottleneck and enables voltage conversion efficiency close to the theoretical limit. Using gallium oxide (Ga₂O₃), we successfully fabricate this phonon-assisted Ga₂O₃ PCSS, demonstrating a voltage conversion efficiency of 90.25% at an excitation laser energy of 1.98 mJ and a bias voltage of 600 V, as well as a conversion efficiency of 98.93% and a maximum output power density of 17.7 MW/cm² at a bias voltage of 4000 V. This work provides a new principle and technical route for designing and fabricating high-performance high-power photoconductive switches, significantly advancing the development of high-efficiency, high-power microwave systems.