In order to identify a method suitable for describing the Relative Wavelength Response (RWR) in the sine wave scanning wavelength modulation spectrum, and to improve the accuracy of the sinusoidal scanning measurement of gas concentration, existing methods for describing the RWR of the laser are discussed. First, several existing methods for describing the wavelength response of a laser for sinusoidal scanning are analyzed. Then, the RWR simulated by the different methods and the residuals of the measured laser wavelength response are compared. The smaller the residual, the more accurate the wavelength response of the laser. Finally, the carbon dioxide gas concentration is measured in a precise and accurate manner. The experimental results show that the method of directly determining the RWR by the current scanning wavelength response describes the laser wavelength response more accurately. The RWR obtained is compared with the results measured by the etalon, and the standard deviation of the residuals is less than 5×10-3 cm-1. By combining this method with multichannel cell technology and calibration-free wavelength modulation technology, the accuracy of the method was verified by measuring the concentration of carbon dioxide gas.