Abstract: Frequency modulation tunable diode laser absorption spectroscopy (FM-TDLAS) is a high-sensitivity trace gas detection technique. In order to improve the signal to noise ratio（SNR）of a methane gas detection system, and to enhance detection sensitivity, a high-frequency phase-sensitive detector with Double Balanced Mixer (DBM) as the core and a modulation circuit were designed to detect the small signals in strong noise. Based on the design and analysis of the high-frequency phase-sensitive detector and the modulation circuit of the system, the performance testing for the overall circuit in the FM-TDLAS system was carried out in the experiment, in which the near-infrared absorption spectrum of CH4 gas around 1653nm was utilized. The experiment shows that the first harmonic and second harmonic signals with high SNR are obtained, and the detection sensitivity reaches 10 ×10-6.