The peculiar photoconduction in semi-insulating GaAs photoconductive switch being triggered by 1064nm laser pulse is reported.The gap between two electrodes of the switch is 4mm.When it is triggered by laser pulse with energy of 0.8mJ and the pulse width of 5ns,and operated at biased electric field of 2.0 and 6.0kV／cm,both linear and nonlinear modes of the switch are observed respectively.Whereas the biased electric field adds to 9.5kV／cm,and the triggered laser is in range of 0.5～1.0mJ,the peculiar performed characteristic is observed:the switch gives a linear waveform firstly,and then after a delaytime of about 20～250ns,it outputs a nonlinear waveform again.The physical mechanism of this specific phenomenon is associated with the anti-sitedefects of semi-insulating GaAs and two-step-single-photon absorption.The delay time between linear waveform and nonlinear waveform is calculated,and the result matches the experiments.