The availability of continuouswave tunable light source in the midinfrared region is valuable for atmospheric sounding, industrial production, medical diagnosis, etc. 1 060 and 1 550 nm band narrow linewidth laser is used as the fundamental light source, the midinfrared light source based on difference frequency generation (DFG) and quasiphasematching (QPM) technique has been realized in a periodically poled LiNbO3 (PPLN) crystal. The conversion efficiency, the temperature and the wavelength tuning characteristic of the midinfrared light source are studied experimentally. Based on the results described above, a gas detection system based on the midinfrared DFG laser source is constructed and the absorption spectrum at 2 999.01 cm-1 of v3 fundamental vibrational band of CH4 molecules is measured by scanning the wavelength of the pump laser.