Wavelength modulation-direct absorption spectroscopy (WM-DAS) integrates the advantages of measuring absolute absorbance profile from calibration-free direct absorption spectrum (DAS) with the enhanced noise rejection and high sensitivity of wavelength modulation spectrum (WMS). This method can be used to precisely recover the crucial absorbance profile via the extraction of the characteristic frequency of the modulated transmitted light. In this paper, the WM-DAS method with non-calibration and high signal-to-noise ratio is integrated with a Herriott cell (about 128 m). Under the condition of atmospheric pressure and room temperature, the absorptance functions of two spectral lines of CO₂ (6330.821 cm^–1) and CH₄ (6046.964 cm^–1) in air are measured, and their standard deviations of spectral fitting residual are 5.6 × 10^–5 and 7 × 10^–5, respectively. Subsequently, the concentration of CO₂ and CH₄ in air are monitored on-line by the WM-DAS method integrated with the Herriott cell, and compared with those by the highly sensitive continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results show that the measured results of the long optical path WM-DAS method are consistent with those by the CW-CRDS method, and the linear correlation between the two methods is above 0.99. The detection limit of CO₂ and CH₄ by the WM-DAS method are 170 ppb and 1.5 ppb respectively, which are slightly higher than those by the CW-CRDS. However, the measurement speed by WM-DAS is much higher than that by CW-CRDS, and possesses the advantages of simpler operation, lower environmental requirements, long-term stability, etc.