The effect of bias voltage on the characteristic of GaAs photoconductive semiconductor switches (PCSS) was investigated theoretically and experimentally. The outputs of the switches for different bias voltages were obtained by solving the basic equations of transient model of PCSS. With a bias voltage of 2400V and triggered by a laser diode, the high gain PCSS switched a electric pulse with voltage up to 1700V. The simulated results agree with the experiment observations well. A new phenomenon of carriers accumulation effect was found.