A 4 mm gap semi-insulating(SI) GaAs photoconductive switch(PCSS) was triggered by a pulse laser with a wavelength of 1064 nm and a pulse energy of 0.5 mJ.In the experiment,when the bias field was 4 kV, the switch did not induce self-maintained discharge but worked in nonlinear(lock-on) mode.The phenomenon is analyzed as follows:an exciton effect contributes to photoconduction in the generation and dissociation of excitons. Collision ionization,avalanche multiplication and the exciton effect can supply carrier concentration and energy when an outside light source was removed.Under the combined influence of these factors,the SI-GaAs PCSS develops into self-maintained discharge rather than just in the light-controlled prebreakdown status.The characteristics of the filament affect the degree of damage to the switch. A 4 mm gap semi-insulating (SI) GaAs photoconductive switch (PCSS) was triggered by a pulse laser with a wavelength of 1064 nm and a pulse energy of 0.5 mJ. The experiment, when the bias field was 4 kV, the switch did not induce self-maintained discharge but worked in nonlinear (lock-on) mode. The phenomenon is analyzed as follows: an exciton effect contributes to photoconduction in the generation and dissociation of excitons. Collision ionization, avalanche multiplication and the exciton effect can supply carrier concentration and energy when an outside light source was removed. Under the combined influence of these factors, the SI-GaAs PCSS develops into self-maintained discharge rather than just in the light-controlled prebreakdown status. of damage to the switch.