Photoconductive semiconductor switch is of significance in the fields of ultafast electronics, high-repetition rate and high-power pulse power system, and THz radiation. The mechanism of the nonlinear mode of the switch is an important area of study. In this work, stable nonlinear wave forms are obtained by a semi-insulating GaAs photoconductive semiconductor switch triggered by a 5-ns laser pulse with pulsed energy of 1 mJ at a wavelength of 1064 nm under a bias of 2750 V. Based on two-photon absorption model, the photogenerated carrier concentration is calculated. The theory analysis and calculation result show that the photogenerated carrier can compensate for the lack of intrinsic carrier, and lead to the nucleation of photo-activated charge domain. According to transferred-electron effect principium, the electric field inside and outside the domain are calculated, indicating that the electric field within the domain can reach the electric field which is much larger than intrinsic breakdown electric field of GaAs material, and results in strong impact avalanche ionization in the bulk of the GaAs switch. According to the avalanche space charge domain, the typical experimental phenomena of nonlinear mode for GaAs switch are analyzed and calculated, the analysis and calculations are in excellent agreement with the experimental results. Based on drift-diffusion model and negative differential conductivity effect, the transient electric field in the bulk of the switch is simulated numerically under the optical triggering condition. The simulation results show that there are moving multiple charge domains with a peak electric filed as high as the intrinsic breakdown electric field of GaAs within the switch. This work provides the experimental evidence and theoretical support for studying the generation mechanism of the nonlinear photoconductive semiconductor switch and the improvement of the photo-activated charge domain theory.