In this study, a coupled MM5-CAMx air quality modeling system was proposed for simulating the SO2 concentration in Beijing, China during the heating season. The Particulate Source Apportionment Technology (PSAT) was introduced into the modeling system to investigate SO2 source apportionment from regions nearby Beijing and regions in Beijing. The results indicated that the influence from regions outside of Beijing including Hebei, Tianjin and Shanxi could not be ignored, and the total Emission Source Contribution Ratio (ESCR) from these regions was 10.8%. Metropolitan Beijing, including Chaoyang, Fengtai, the city center of Beijing, Haidian, and Shijingshan, with an ESCR of 82.5%, was the main contributing area in Beijing. The results obtained could provide useful information for Beijing’s SO2-emission control and abatement. This method could effectively identify SO2 pollution problem in a metropolis region and thus provide a scientific basis for air pollution control and mitigation.