Combining cavity ring down spectroscopy (CRDS) and wavelength modulated direct absorption spectroscopy (WM-DAS), a wide range and calibration-free gas concentration detection method is established, which has the advantages of high signal-to-noise ratio of CRDS and fast speed and measurable absolute concentration of WM-DAS. The baseline ring down time (τ₀) can be calculated by measuring the absorptivity (WM-DAS) and ring down time (CRDS) at the central frequency of the spectral line, without real-time calibration, which greatly improves the speed of CRDS measurement. The measurement results of different H₂S concentrations at 6336.617 cm^–1 at room temperature and atmospheric pressure show that the dynamic measurement range of this method can be extended to more than 4 orders of magnitude, the measurement accuracy is improved in comparison with WM-DAS, and the detection limit can reach 1 × 10^–9 in 40 s.