Our recently proposed three-step method showed the promising potential to improve the accuracy of relative wave-length response(RWR)characterization in the wavelength-modulation spectroscopy(WMS)over the commonly used sum-mation method.A detailed comparison of the three-step method and the summation method,for the wavelength-scanned WMS gas-sensing,was performed with different laser parameters(modulation indexes and scan indexes)and gas proper-ties(pressures and concentrations).Simulation results show that the accuracy of the predicted gas parameters is strongly limited by the RWR characterization with large modulation index and high gas pressure conditions.Both fitting residuals of RWR and errors of predicted gas parameters from the recently proposed three-step method are nearly 2 orders of magni-tude smaller than those from the summation method.In addition,the three-step method is further improved by introducing a coupling term for the 2nd harmonic amplitude.Experiments with CO2 absorption transition at 6976.2026 cm-1 were conducted and validated the simulation analysis.The modified-three-step method presents an improved accuracy in RWR description with at least 5％smaller fitting residual for all conditions compared with the three-step method,although the deviation of the deduced CO2 concentrations between these two methods does not exceed 0.2％.