The beam spot is one of the important factors affecting the on-state performance of photoconductive switches. The on-state performance of the GaN photoconductive switch has been tested under the triggering of Gaussian beam and flat-top beam. As the energy uniformity of flat-top beam is better than that of Gaussian beam, the results show that the voltage conversion efficiency is increased by 6.8% under the same applied bias voltage (800 V), Triggered by flat-top beam at a laser energy of 500 μJ, GaN PCSS shows a maximum peak output voltage of 4550 V, at the same time the output power reaches 414 kW and the conduction is 13.7 Ω. The output waveform has a rise time of 420 ps and a fall time of 5 ns.