Abstract: In order to solve the insufficient of signal process algorithms during the detection of atmospheric trace gas concentration by wavelength modulation laser spectroscopy technique, a new method of gas concentration inversion based on the random sample consistency (RANSAC) algorithm was proposed. By choosing the simulation signal and the actual measurement signal of formaldehyde in the atmosphere as examples, theoretical analysis and experimental study were carried out and compared with the traditional least square method. The results show that the proposed algorithm has better immunity to noises and outliers. Especially under the conditions of low signal-to-noise ratio (SNR), the measurement accuracy can be improved by one order of magnitude. The algorithm shows better reliability and superiority.