Results of experiments of the 4 mm gap semi-insulating(SI) GaAs photoconductive switch triggered by 1064 nm, 1.0 mJ pulse laser showed the nonlinear mode when the bias field was 3800 V. Under the same bias electric field and trigger light energy conditions, the switch outputs stably nonlinear electrical pulses, and the switch surface injury mark is caused by filamentation after 1500 times triggering. Analysis shows that under given conditions of trigger energy and electric field, two transient thermal effects occur in the switch chip, namely the thermal relaxation and photoactivated charge domain-phonon drag, respectively. Thermal relaxation time is shortened to the order of picoseconds or subpicoseconds, thermal relaxation process leads to the thermal conduction relaxation. When photoactivated charge domain moves at 107cm/s speed from cathode to anode, switch chip transient temperature makes relaxation oscillations owing to these effects, and the rapid increase of temperature in the chip is constrained. Photoactivated charge domain-phonon drag effect transmits in the direction of the dislocation movement, the temperature in mobile region increases when the flow of thermal energy carried by the phonons was concentrated in the movement plane, the injury of filamentation is produced by superposition and cumulation of mobile tracks.