Abstract: In order to investigate the output stability of Q-switched diode-pumped laser (DPL) pulse, the tradition detection circuit of the silicon photodiode was analyzed with the circuit mesh method. It is found that the traditional detection circuit is unable to detect the high frequency noise signal generated by the multi-longitudinal modes in the laser pulse. According to the analysis result, the silicon photodiode detection circuit was improved, and (1～2)GHz high frequency noise signal of a Q-switched laser was collected by the high speed sampling oscilloscope.