Abstract: Nonlinear absorption occurs when the differential optical absorption spectroscopy (DOAS) is used to monitor the flue gas with high SO2 concentration from fixed pollution source. A nonlinear compensation method is put forward, in which the ratio of the pressure component of the actual concentration to the pressure component of the inverted result versus the inversed result is fitted into a compensation function. The compensation function is used to compensate the inverted result nonlinearly. Experimental results show that the effect caused by the non-linear absorption of highconcentration gas can be reduced with the proposed method and it can improve the inversion accuracy of gas concentration.