In order to detect optical signals in very low and high optical power range, a weak light detection system based on silicon photomultiplier (SiPM) is proposed. This system consists of auto-adjusting bias circuit, current-voltage conversion circuit, small signal amplification circuit and filter circuit. The relationship, between the output of SiPM and the incident light signal under different bias voltage was measured. The experimental results show that the bias has a great influence on the output of SiPM, the detection ability and detection range of SiPM under different bias voltage. In addition, the system has response of the 25 pW to 1.75 μW optical signals, which can achieve continuous detection of optical signals in very low and high optical power range.