A heavy air pollution episode in Beijing in October 2015 was analyzed through vertical observation, ground-based observation, monitoring data of chemical components of PM_2.5 and back-trajectories of air mass. The results showed that aerosol extinction coefficient increased significantly during the episode, indicating that air pollutants accumulated within 600m height from ground. Comparing with normal days, wind speed was weaker, relative humidity was higher, surface pressure was lower and boundary layer height was lower during the episode. The variation and peak appearance were consistent among different monitoring sites, and NO₃^- concentration was higher than other components of PM_2.5 in most time. In addition, the surrounding region of North China Plain affected by heavy pollution was relatively small, while PM_2.5 accumulation in Beijing and its nearby region was more prominent. Multi-observations and correlation analysis between PM_2.5 and meteorological factors and chemical compositions showed that this episode was affected by regional transport to some extent, including straw burning. However, the accumulation and chemical reaction of local NO₂ emission under unfavorable meteorological conditions and extreme space compression in vertical direction was the main cause.