By using laser diode with low energy to trigger GaAs Photoconductive Semiconductor Switch (PCSS) at high bias voltage, which makes GaAs PCSS working under avalanche mode, fast pulse voltage is produced. GaAs PCSS is designed as vertical bulk structure with 2 mm thickness. The electrodes are of round and doughnut configuration and the trigger light passes through the center of doughnut electrode. The fast pulse is achieved by coaxial Blumlein pulse form line. The simulation and experiment of coaxial Blumlein pulse form line based on GaAs PCSS is executed. When charge voltage exceeds 8 kV(40 kV/cm), the output voltage impulse increases rapidly and the rise time is shorter than that of laser impulse, which indicates PCSS turns into avalanche mode. When bias voltage is up to 15 kV(75 kV/cm), 11 kV voltage pulse is obtained under 50 Ω of load, which accords well with the simulation result. The switch jitter is tested, and the results show that the switch bias voltage has great influence on the switch jitter. As the switch bias voltage increases, the switch jitter decreases. When the bias voltage reaches 15 kV, the lowest jitter of 700 ps is obtained.