Abstract: Gas spectroscopy detection based on mid-infrared laser source has been applied in the field of atmospheric monitoring as a new method for measuring and analyzing trace gases.A detection system of methane gas based on mid-infrared difference-frequency generation (DFG) laser source was constructed.The system uses a tunable semiconductor laser with 1 550 nm and 1 060 nm band as the fundamental frequency source, and uses a periodically-poled lithium niobate (PPLN) crystal as the differential frequency nonlinear inverter to achieve a narrow linewidth tunable mid-infrared source output at 3.3 μm. Experimental results show that, when the temperature of the PPLN crystal is fixed at 99.5℃, the maximum idler laser output power of 112 μW, and the nonlinear DFG conversion efficiency is 1.246 mW/W². The acceptance bandwidth for the pump wavelength and the crystal temperature are 5.3 nm and 4.3℃, respectively. On this basis, the methane gas absorption spectrum and the second harmonic detection signal at 3 028.751 cm^-1 were obtained by direct absorption method and harmonic detection method, respectively.