In the lock-on model of semi-insulating GaAs photoconductive switches，the output current pulse has ultra-fast rise character，the rise time can be even less than that of the laser pulse. Fluid model is used to simulate the transport of photo-induced carrier. The results show that photo-activated charge domain formed in the transport process of carriers is the main cause of ultra-fast rise time. Further studies on the establishment of photo-activated charge domain and the absorption process show：1. The photo-activated charge domains during the establishment enhance the electron density gradient. 2. When the photo-activated charge domains are absorbed by the anode，rapid decline of bias electric voltage will accelerate the rise rate of output current pulse. Due to the two processes, the output current pulse in the lock-on model shows ultra-fast rise character.