The Weather Research and Forecasting model coupled to Chemistry (WRF-Chem model) was modified with an online primary PM2.5 source-apportionment method to simulate a severe aerosol pollution episode that occurred over the Beijing-Tianjin-Hebei (BTH) region of China from October 29 to November 8, 2015. The temporal and spatial distributions and transport characteristics of this episode were examined, and the quantified primary PM2.5 contributions by various geographical source regions to the BTH region were also analyzed in this study. The results showed that pollution in this region was mainly due to the combined effects of the synoptic conditions, terrain, and boundary layer characteristics. Before the heavy pollution event, the most parts of the BTH region were controlled by northwest wind, and the local primary PM2.5 contribution to the BTH region accounted for 90.7%. During the heavy pollution period, the BTH region was under isobaric synoptic conditions with light horizontal winds and a stable temperature stratification structure and a lower PBLH, allowing pollutants to accumulate easily in the region. Large quantities of pollutants were transported to the BTH region from Shandong, Jiangsu, Anhui, and Henan provinces via the southerly wind, accounting for 11.6%, 9.2%, 6.7%, 11.8%, respectively. Furthermore, pollutants accumulated more significantly under a stable boundary layer structure and the northwest-southeast terrain structure. Therefore, regional contributions increased, and other polluted areas, especially the long-distance transport from a source in the Yangtze River Delta city group region, contributed about 15.9%. It should be noted that the percentage of primary PM2.5 within the total PM2.5 in the study period almost exceeded 50%, and in the BTH region nearly surpassed 60%. To a certain extent, primary PM2.5 can help characterize total PM2.5.