The traditional optical gas detection method needs to convert the time domain signal to frequency domain signal to obtain the concentration of gas, which will introduce errors in the conversion process and cannot accurately reflect the change of frequency with time, resulting in the gas concentration cannot be accurately calibrated. A detection method based on frequency domain absorption peaks to calibrate gas concentration was proposed. Taking CO₂ as an example for detection, the concentration of CO₂ in the mixed gas of CO₂ and N₂ was determined by using the direct absorption method based on the Beer-Lambert law. This method obtained the peak intensity of the absorption spectrum by collecting the light intensity and frequency information of the laser beam and performing the Lorentz fitting by the adjacent averaging method and Levenberg-Marquardt (LM) optimization algorithm. The experimental results show that the linear relationship between laser peak intensity and gas concentration is satisfied, and the Pearson correlation coefficient can reach 0.999. Therefore, the detection method based on the frequency domain absorption peak calibration of gas concentration can effectively simplify the system structure, avoid the effect of frequency drift in the time domain to frequency domain conversion, and make the linearity between laser peak intensity and gas concentration good.