Multi-method observation and numerical simulation were applied to analyze a PM2.5 pollution episode in Beijing in October, 2014. The results of vertical observation showed that surface-level backscatter signal and extinction coefficient increased during the episode, suggesting that air pollutants accumulated near the ground. The main meteorological factors during this episode could be described as calm wind, high relative humidity and low surface pressure. The evolution of PM2.5 concentrations in this episode was divided into four stages, including two-steps type concentration climbing stages (P1 and P2), high concentration maintenance stage (P3) and rapid cleanup stage (P4). Analysis on ground-based observation, satellite remote sensing and atmospheric general circulation showed that regional transport, including crop residue burning, was the main incentive of this pollution episode. Subsequently, local pollutants emission and regional transport maintained and aggravated the episode under unfavorable meteorological conditions. Temporal variation of OX was in close agreement with that of PM2.5 and the concentration peaks of OX occurred few hours before those of PM2.5, which indicated that strong atmospheric oxidation could promote the formation of secondary PM2.5. The results of numerical simulation showed that during 8–10 October, the average contribution of regional transport to PM2.5 in the five sites exceeded 50%.