In order to detect trace methane (CH4) with non-contact method, a trace CH4 detector is designed and developed with the combination of tunable diode laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS) detection technology, using the absorption line (1 332.8 cm-1) of CH4 in mid-infrared band. The instrument uses mid-infrared quantum cascaded laser (QCL) with centre wavelength at 7.5 μm, and uses the method of changing the injecting current (0.6～1.6 A) of QCL with fixed working temperature to make the emission wavelength of QCL to scan the methane’s absorption line (1 332.8 cm-1) via tuning parameters 0.2 cm-1·A-1. In terms of optical structure, the instrument using a gas absorption sealed herriott cell with 76 m long optical path, cooperating with difference detection optical path, reduces the noise which caused by the fluctuation of QCL, and guarantees the detection of trace CH4. In the experiment, we adopted minimum mean square error criterion to fit the relationship between methane concentration and harmonic peak signal. In addition, the minimum detection limit is 40×10-9, and the relative error of test results is 0.09%., The stability is better than 2.8%, which verify the feasibility of the instrument.