We report on the experimental results of a lateral semi-insulating GaAs photoconductive switch, with a gap of 8mm between two electrodes, triggered by 1064nm laser pulses at a wavelength beyond the GaAs absorption edge. Both the linear and nonlinear modes of the switch were observed when it was triggered by light pulses with an energy of l.9mJ and a pulse width of 60ns, and operated at high voltages of 3 and 5kV. The results show that when the semi-insulating GaAs photoconductive switch operates under an electrical field of 4.37 k V/cm, it will enter into the linear mode first, and then the switch will undergo the nonlinear mode (lock-on) after a delay of about 20 - 100 ns. It is worth noticing that the delay time under high light energy is longer than that in the low optical energy. The non-intrinsic absorption mechanism is discussed. EL2 deep level defects and double-photon absorption in GaAs may play a key part in the absorption process.