Cavity dumping is an effective technique for generating Q-switched laser pulses of relatively large energy and extremely short time duration, and the width of Q-switched pulses is primarily a function of the oscillator cavity length. The solid state lateral semi-insulating GaAs photoconductive semiconductor switch (PCSS) has the unique ability to handle high power at very fast response time with very little timing jitter. A novel and effective technique of cavity dumping laser for generating short Q-switched laser pulses with a GaAs PCSS is presented. In a flashlamp pumped Nd:YAG laser with 20 cm long stable resonator, 1.7 ns short laser pulses have been obtained, and the pulse to pulse duration instability is less than 7% and energy instability is less than 3%.