The leakage of methane and sulfureted hydrogen in the tunnel may threaten the environment, safety and lead to damage and casualties. In this paper, a system based on the distribution feedback laser is introduced to detect the leakage natural gas in the tunnel. The system can be used in the tunnel with the length over 2km. In the system, two approaches, namely the direct absorption and wavelength modulation is both adopted to acquire the concentration of methane and sulfureted hydrogen. The results show that the remote sensing can reach up to two kilometers with the detectability of methane 2ppm and sulfureted hydrogen 20ppm respectively. This system is carried out in the Puguang Gas filed for the first time and satisfactory results are obtained, making up the gap in such field as detecting natural gas in domestic market.