Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace gas qualitively or quantitavely, based on the tunable characteristic of the diode laser, by obtaing the absorption spectroscopy in the characteristic absorption region. For the significant advantage in sensitivity， selectivity and rapidity of response, it has been widely used in atmospheric trace gas measurements and industrial control. In air trace gas measurements, TDLAS should be combined with the long absorption path technique due to the low concentration. Based on the combination of TDLAS and 27 m absorption path after 108 times reflection a portable absorption spectrometer is developed for monitoring the ambient methane, and the detection limit is below 1×10-7 by second harmonic detection which is usually used to detect low signals. The system is tested by the gas with different concentration， and the result is very well.