In recent years, the eastern part of China, especially the JingJinJi (shortly for BeijingTianjinHebei Region) has suffered increasingly severe pollution of PM2.5. A case of PM2.5 pollution over the JingJinJi Region during the autumn of 2014 is studied with WRFChem model in this paper. The results show that the pollution was caused by a stable weakhigh pressure system with the dominant southerly wind on the surface. The temperature inversion layer further suppressed the diffusion of pollutants in the vertical direction. High concentration of PM2.5 mainly spread in southern Beijing, Tianjin and northern Hebei Province, where the contribution of secondary PM2.5 was higher than that of primary PM2.5 to most of the pollution areas. Vertically, the secondary inorganic particles accumulated around Beijing, then became a high concentration region affixed to the ground surface. On the other hand, there was a zone of high pollutants over 1.2-1.6 km, and primary pollutants only had one high value area near the surface. The vertical distribution height of PM2.5 showed a similar trend with the boundary layer height.