The measurement accuracy of the pulsed laser rangefinder is decreased due to the background light interference of the photodetector. Combined with the application environment of laser ranging system, a model of photodetector under background light is founded, and the influence of background light interference on the detection system is also analyzed. Based on that, the part of the launch, receiving, optical was targeted designed respectively to achieve effects of background suppression. Also, a new method is also proposed to simplify the launching time discrimination circuit, which not only reduces the circuit structure, but also easy to implement. The result show that, the system can effectively avoid the interference of the background light and improve the stability of the system under the condition of meeting the design requirements, and its ranging accuracy is <5 cm.