This paper proposes a calibration-free wavelength modulation spectroscopy (WMS) method based on the height-width characteristic of the second harmonic and gives the analytical expression of the height and width of the second harmonic for the spectral adsorption factor considering the Voigt line-shape function under arbitrary modulation index. Compared with the traditional harmonic waveform fitting method, this method only requires filtering processing once and has lower requirements for computing resources. Instead of using spectral parameters such as collisional broadening coefficients and temperature-dependent indexes from the HITRAN 2020 database, it is theoretically more suitable for measurements under complex gas components. A WMS measurement system is set up in the laboratory to measure the mole fraction of CH₄ at room temperature. The experimental results reveal that when the mole fraction of CH₄ is greater than 2.08×10^-3 under the 20 cm optical path, the relative deviation of the proposed method is less than 2%.