Pollution characteristics processes and potential sources of PM_2.5 were studied in a severe haze episodes in Beijing from 26 November to 2December, 2015. Pollution characteristics and meteorological parameters were analyzed. Hourly 72-hour backward trajectories in ground (500m) and high altitude (3000m) were classified and the effect of clusters in ground and high altitude on PM_2.5 were analyzed using HYSPLIT model and cluster method. Major potential sources of PM_2.5 were simulated using Potential Source Contribution Function (PSCF) and Concentration-weighted Trajectory (CWT) methods. The results showed that hourly PM_2.5 concentration in Beijing varied widely and low temperature, high humidity and low wind speed provided a suitable condition for the heavily pollution process of PM_2.5. Airflow trajectories from different directions had an significant impact on the spatial distribution of PM_2.5 in Beijing. Air trajectories from northwestern were the dominant trajectories which had a big influence on PM_2.5 concentration in Beijing. Moreover, air trajectories in ground from southern should not be ignored because air trajectories that passed through southern areas carried anthropogenic pollutants to Beijing. WPSCF (weighted potential source contribution function) and WCWT (weighted concentration-weighted trajectory) values were more than 0.7 and 200μg/m³ outside of Beijing, respectively, which revealed that mid-western Mongolia, eastern Xinjiang, mid-western Inner Mongolia, northern Shanxi, Hebei and regions in northern Shandong were the major potential sources of PM_2.5 in Beijing.