This paper mainly focuses on the concentration calculation of SO2 in flue gas. There are many methods for smoke detection. First of all, this paper mainly introduces the basic Differential Optical Absorption Spectroscopy technology. Then, in order to solve the problem of overlapping absorption between different gases in the flue gas, this paper use cyclic iteration algorithm to measure the concentration of SO2. It is concluded that the basic DOAS algorithm has a large error for the concentration measurement of mixed gas with overlapping absorption, the he error is above 25%. However, the cyclic iterative algorithm has a good effect on processing with it which can get a more stable result after three iterations, and the measurement error can be controlled finally within 2%.