In order to monitor SO2 emission from coalfired power factories and meet national ultralow emission standards, measurement method for lowconcentration SO2 via short optical length is proposed. At known pressure and temperature conditions, dataset of differential absorption cross section was constructed with various concentrations of standard gas, utilizing differential optical absorption spectroscopy (DOAS). In accordance with standard deviation and average of the dataset, combined with results of concentration inversion, spectral window was gradually reduced from 200~400 nm to 294~308 nm. For each point in 294~308 nm, where there are 137 sampling points, statistics and standard deviation were applied to characterize consistency of differential absorption cross section. It was only when standard deviation and average at sampling point met requirements that the point was reserved to optimal point set. So optimal dataset of differential absorption cross section and optimal sampling points were constructed eventually. At same pressure and temperature conditions, SO2 concentration in flue gases can be accurately calculated by the optimal dataset and DOAS. In the laboratory, measurement range is 2~30 μL/L, temperature and pressure are 299.05 K, 101.33 kPa, while optical length and volume of gas cell are 420 cm, 0.5 L correspondingly. Experimental results show that inversion concentration performs superior repeatability within 72 h, relative error is less than 1.7%, full scale error is below 1.3%, and zero drift is 0.09 μL/L. With 420 cm optical length, the method can precisely measure lowconcentration SO2 under 30 μL/L, resolve conflicts between concentration and optical length, which is suitable for study of ultralow emission monitoring system used in coalfired power plants.