The methodology is described for detection of SO₂ in flue gases by differential optical absorption spectroscopy(DOAS). The basic principle and data analysis method are introduced. The selection of wavelength range during the measurement is also discussed. Measurement of SO₂ was carried out at different spectral resolution(0.6 nm, 0.7 nm, 1.8 nm) respectively. The influence of spectral resolution on the result was analyzed. DOAS method analyzes high-frequency part of absorption spectroscopy, and it can avoid influences of the interferences such as fume, vapor. It is proved to be a good method for detecting the flue gas.