Abstract: The atmospheric PM_2.5 samples in urban of Beijing were collected in January and July, 2015, which represented the winter and summer respectively. The samples were analyzed for the mass concentration changes and pollution characteristics of PM_2.5, organic carbon (OC) and elemental carbon (EC), combining with relevant meteorological data. Moreover, the mass concentrations of secondary organic carbon (SOC) were estimated by ρ(OC)/ρ(EC) minimum ratio method, and the effect of air mass transport on haze formation was studied by using of backward trajectory and cluster analysis model. Results showed that the mass concentrations of PM_2.5 and carbonaceous aerosol were expressed as winter > summer, haze weather > non-haze weather. SOC is an important part of OC, accounting for 47.16% in winter, 55.54% in summer, respectively. The air mass trajectories in haze weather in winter were mainly controlled by northwest high air mass and local air mass, among which the local air mass transport from Beijing-Tianjin-Hebei (BTH) region was a great contribution to the haze pollution. While, the air mass trajectories in haze weather in summer were mainly affected by southeast air mass, northwest air mass and southwest air mass. The frequency of air mass trajectories from the south was higher and have a greater contribution to haze pollution. Therefore, strengthening the air pollution joint prevention and control in Beijing-Tianjin-Hebei and its surrounding areas is of great significance for improving air quality in Beijing.