It is difficult to realize real-time power detection as the upstream signal of gigabit passive optical network(GPON) system is high-speed burst mode. Based on the analysis of the upstream signal transmission characteristics, the frame overhead, and the noise model, a new detection method is presented. This method raises the signal to the original system with pre-through high-impedance amplifier, which does not affect the performance of the original system. High-frequency burst signal will be down converted for processing, through the peak detecting and low-pass filtering. A limiting amplifier and digital decision device of low-level optical receiver module can be used to correct the error after the signal′s average power is obtained. Since the method can improve the signal-to-noise ratio, and make full use of system function to obtain information of the duty ratio, it is low-cost and has high detection accuracy. The experimental result has shown that the method is quite suitable for GPON system upstream signal power detection.