The 4 mm gap and 5 ns pulse width semi-insulating GaAs photoconductive switches were triggered by 532 nm laser pulse with gradual increase of bias voltage from 500 V in steps of 50 V until the emergence of nonlinear electrical pulse. The expermental results showed that the linear and nonlinear electrical pulse waveforms had smaller amplitude and varying degrees of oscillation reduction after going through a main pulse. Then the microscopic state and transport process of carriers （hot-electron） in the switch material were studied in detail using the quantum theory. It was found that in the DC bias electric field, when the relaxation time of the hot-electrons in the electron-electron and electron-phonon interaction process is longer than the carrier life, the photoconductivity oscillation can be caused by the change of mobility in the process of optoelectronic transport, which is the main cause for the output electrical pulse to show oscillations.