An experimental device of wavelength modulated off-axis integrated cavity output spectroscopy (WM-OA-ICOS) technique is reported. A distributed feedback (DFB) laser with central wavelength of 1.392 μm is chosen as light source. A resonant cavity is composed of two mirrors with reflectivity of 99.8% and 60 cm distance, and the cavity is used as gas absorption cell. The absorption line of CH4 at 7185.87 cm -1 is selected to detect CH4 with different concentrations. The stability time of the system is obtained to be 203 s by optimizing the instrument parameters (pressure, modulation frequency, phase and amplitude) combining with Allan variance. The detection limit of CH4 is obtained to be 8.7×10 -7 by selecting the measurement time of 100 s, and the corresponding minimum detectable absorption is 2.2×10 -6 Hz -1/2. Compared with that of off-axis integrated cavity output spectroscopy technique, the sensitivity of WM-OA-ICOS technique increases by 21 times. The second harmonic peak height and the ratio of the peak height of second harmonic to the medium of first harmonic (2f/1f) are used to measure the concentration of CH4, and the results show that the latter has better stability and higher degree of linearity correlation.