Semiconductor laser diode triggered GaAs Photoconductive Semiconductor Switch(PCSS) works in avalanche mode, therefore, a vertical GaAs PCSS with bulk structure is fabricated to enhance the switch field. The proposed switch is of 2 mm thickness and 3 mm gap, and triggered by laser diode. When charge voltage exceeds 8 kV, the amplitude of output voltage impulse increases rapidly, and the output impulse front is faster than that of the laser impulse, which shows PCSS turning into avalanche mode. Along with the enhancement of switch electric field, the output voltage increases linearly, nevertheless, there are no changes on the output waveform. The switch jitter is tested, and the results show that switch bias voltage has great influence on switch jitter. Along with the increasing of switch bias voltage, the switch jitter reduces; and when bias voltage up to 15 kV, the lowest jitter of 500 ps is obtained.