III V compound semiconductors, such as Indium phosphide (InP) and its doped materials, are very suitable to make the transient high power photoconductive semiconductor switches (PCSSs) with its picosecond (ps) time response. An InP PCSS is fabricated, and measured under the input voltage up to 2000 V and the optical energy up to 2000 uJ conditions. The experimental data are explained by using a simple model. It is shown that the output voltages increase initially with the increasing optical energy at the initial stage, but saturate when the optical energy is big enough. The output voltages almost increase linearly with the increasing input voltage under the low optical energy condition, but deviate slightly linear relationship when the optical energy is high. Therefore, this paper proves a light-controlled photoelectronic device which could provide stable ps-order pulse. And this device could be the power source of the ultra-fast electric product.