This article carried theoretical analysis and experimental research on the scheme of remote sensing the minimum detection limit concentration for methane gas using a single laser source. In this project, the frequency-modulation (FM) and harmonic detection (HD) technologies are adopted. It is obtained through the theoretical calculation that the minimum detection limit concentration for methane gas was about 8.7×10-8 m. Simultaneously the experimental sensitivity of the detection system and the minimum measurable path-integrated concentration were measured to be 8.43×10-6 m/mV and 4.2×10-7 m respectively. This results show that the remote sensing system owns a super-high sensitivity and it can satisfy the need of monitoring methane in the coal mine.