In this paper, we propose a method of measuring a gas by using non-dispersive infrared (NDIR) technique with two analysis channels. The filter parameters of the two analysis channels are calculated by line-by-line integral of the selected absorption spectrum of SO2. The influences of temperature and air pressure on strengths and Lorentzian shape functions are considered accurately in the calculation. The absorption wavelengths at 7.32 μm and 4.0 μm are chosen to detect the SO2 whose concentrations are ≤qslant 280 ppm and > 280 ppm, respectively. The calibration curves of the two analysis channels are obtained by least-squares fitting two 3-order polynomials. The linearity, the sensitivity and the accuracy of the analysis system are analyzed. SO2 concentration with a large range from ～ 5 ppm to 10000 ppm can be retrieved with the measurement linearity > 0.99 and measurement error < 5%. The reasonable tradeoff is made to optimize both sensitivity and measurement range jointly. A fair balance between measurement sensitivity and large span range is obtained. Furthermore, sufficiently good measurement linearity makes cross-interference correction possible in the NDIR multi-gas analyzer.