In this paper, the wavelength modulation spectroscopy (WMS) technique is modified and used for measuring methane with large absorbance. The WMS has been frequently used for gas measurement and relies on the linear relationship between the second harmonic amplitude and the gas volume concentration. However, the conventional WMS technique is only applicable for the gas whose absorbance is much smaller than 1, which is because the first-order approximation of Lambert-Beer's law is required in the derivation of the traditional WMS theory, and the first-order approximation holds only at low absorbance, hence the linear relationship between the second harmonic and the gas concentration does not hold at large absorbance. In the modified WMS in this work, it is not necessary to make any approximation to Lambert-Beer's law. The measured light is absorbed by the gas to be measured and then collected by the photodetector. The reference light is directly detected by another photodetector without being absorbed. The output signals of the two photodetectors are transmitted to the computer after implementing analog-to-digital conversion. In this way, the demodulated second harmonic signal remains linear with the gas concentration even at large absorbance. In this work, the traditional WMS theory and the modified WMS theory are both introduced, and a series of methane gas with concentration gradients are measured separately. The experimental results of the traditional WMS and the modified WMS are compared with each other. It is confirmed that the linearity in the traditional WMS theory no longer holds under large absorbance, but the improved WMS can still guarantee the linear relationship between the second harmonic and the methane concentration, which verifies the advantages of the modified scheme. Finally, through Allan's standard deviation analysis, the stability of this methane measurement system reaches the optimal value at the average time of 103.6 s, and the corresponding Allan's standard deviation is 1/26.62×10 ^–9volume.