SO2 monitoring in the flue gas of a coal burning boiler is important for environmental protection. The nonlinearity of practical condition causes deviation from theoretical law. On the basis of the Lambert-Beer Law, a new four-step calibration method is introduced. This method includes cross section interpolation, weighting spectral region combination, acquiring the spectrum with new calibration devices, and least-square fitting. Compared with conventional methods, this new method is low cost, convenient, and accurate. In the proof test, SO2 samples with different concentrations are measured. The average errors are less than 1.5%, while the maximum deviation is less than 4.5%.