The availability of room-temperature continuous-wave tunable narrow band coherent light sources emitting in the mid-infrared region is greatly valuable for trace gas sensing techniques. Nonlinear optical difference-frequency generation(DFG)scheme was a useful way to meet the requirement and good complementarities for traditional laser technology. A widely tunable room-temperature mid-infrared spectrometer was reported and the instrument was based on the quasi-phase-matched DFG of two near-infrared diode lasers in multigrating periodically poled LiNbO₃ crystal. The mid-infrared coherent radiation was produced from 3.2 to 3.7 μm with a maximum output power of about 1 μW. The generated mid-infrared coherent radiation was applied to high-resolution detection of CH₄ in the fundamental band,which showed the great promising of the instrument used in sensitive,high-resolution and fast response measurements of trace gas in the mid-infrared region.