Tunnel visibility has an important relationship with traffic safety. Real-time monitoring of tunnel visibility can provide a strong basis for ventilation and ventilation of tunnels. Based on the principle of visibility detection by transmission method, a set of visibility detection system is built for automatic detection of tunnel visibility. The detection system is mainly composed of a laser diode, a beam splitter, a silicon detector and a signal processing unit. Two silicon detectors detect the intensity of light before and after attenuation through optical path respectively and output corresponding photocurrent. The signal processing circuit amplifies and filters the output of the detector and converts the output to AD, and then calculates the extinction coefficient. Fixed transmittance filter is used to simulate tunnel environment, and the performance index of the detection system is tested in laboratory environment. The experimental results show that the measurement range of extinction coefficient is 3.37 km-1～118.82 km-1, the maximum relative measurement error is 8.4%, and the stability is better than 0.12 km-1.