Sulfur dioxide (SO₂) is an unavoidable pollutant product of the fossil fuel combustion process, and its concentration level is considered a key indicator of combustion efficiency and energy utilization, so it is of great significance to realize the monitoring of SO₂ in the combustion process. Absorption spectroscopy technology has shown a wide range of applications in gas detection due to its high accuracy, strong stability, and non-contact measurement. However, a serious nonlinearity between the absorption intensity and the measured concentration of SO₂ at high concentrations occurs in practical measurements, which interferes with the measurement of SO₂ over a wide range of concentrations. To address this problem, an experimental study was conducted to analyze the treatment of nonlinearity in absorption spectra, and an adaptive sliding window absorption spectroscopy (ASWAS) technique was proposed. In the experiment, the sliding window adaptive traversal and selection of the characteristic absorption band, the first and second screening, and the final inversion were performed to obtain the optimal measurement results. Compared with the wide-band and narrow-band methods, ASWAS shows excellent performance with a relative error of less than 1.3% in the concentration range of 50 to 1 500 ppm. The experimental analysis results show that the measurement system has a measurement accuracy of 0.80% and 0.48% at low and high concentrations, and a stability coefficient of 0.28% and 0.21%, respectively. The above experimental results prove that the proposed measurement system based on ASWAS can realize the measurement of SO₂ concentration in a wide range with high precision and high stability, which provides an effective new idea and method for environmental protection and energy utilization.