The Nested Air Quality Prediction Model System (NAQPMS) coupled with an on-line pollutant source tracking module is utilized to estimate the contributions of the air pollutants，i.e., SO2 and PM10，from Beijing and its surrounding areas to urban Beijing under the air quality assurance program for Beijing Olympic Games quantitatively. The results indicate that: 1) The contribution of the local emissions of Beijing is predominant in the SO2 and PM10 concentration in all counties except Pinggu County, especially in urban Beijing, where the contribution of Beijing emissions exceeds 80% of the SO2 and PM10 concentrations. 2) Under the pollutant control measures in the air quality assurance program, the contribution of Beijing emissions to the SO2 concentration in urban Beijing reduced significantly, and that of the surrounding areas(Tianjin and Hebei) to SO2 concentration had a slight decline, thus, the SO2 concentration in urban Beijing decreased significantly, but the contribution per unit emission of Beijing local emissions increased. 3) The contribution of Beijing emission to primary PM10 concentration in urban Beijing decreased significantly while the surrounding contributions had a slight increase, and therefore the PM10 concentration also decreased under the air quality assurance program.