Abstract: Harmonic detection is widely used in laser spectroscopy, which can improve detection sensitivity. In this paper, a set of tunable diode laser absorption spectroscopy(TDLAS) methane detection device is established by using 1.653 μm distributed feedback (DFB) diode laser as light source. The device utilizes two circular cylindrical mirror composed of optical multi-cell to increase absorption of optical path and improve detection sensitivity. Absorption length of multi-pass optical cell is 15 cm, and effective absorption path length is 16.8 m in 112 reflex cases, and detection limit of methane 0.60×10^-6(2 s sampling time) is realized, which can be applied to trace detection of actual atmospheric methane.