In this paper we get the rate equation of photogenerated carriers by solving the rate equation of carriers in a certain
condition. By simulation we draw the general rule of variation of photogenerated carriers' density which depends on
trigger light pulses as well as their width. Also by experimental tests we achieve the effects of photogenerated carriers on
the response speed of Semi-insulating GaAs Photoconductive Switches (PCSS's) and analyze the reasons for the changes
of the rising edge of output electrical pulses.