The inversion accuracy of gas concentration is influenced by the extinction derived from the Mie scattering of dust particles when the differential optical absorption spectroscopy (DOAS) is applied to on-line monitoring exhaustion of fixed pollution source. Simulation and experimental results for particles with diameter of 1～10 μm, show the intensity of particle scattering is determined by particle-size distribution and number concentration. With the increase of particle size and number concentration, the differential optical density of gases rises. Accordingly, the frequency characteristics of the differential optical density curves have also been changed. The influence of dust Mie scattering on differential optical density cannot be eliminated by polynomial filter in traditional DOAS algorithm and the inversion results of gas concentration are greatly deviated from true value.