From 16 to 21December 2016, the Beijing-Tianjin-Hebei region experienced a large-scale heavy pollution process. By using the pollutant concentration monitoring data, weather charts, the synoptic causes of the heavy air pollution were investigated in the present paper. The sources of PM_2.5 pollution in the main cities of Beijing, Tianjin and Hebei region were also analyzed quantitatively by using the Nested Air Quality Prediction Modeling System (NAQPMS). The results showed that it blew straight west wind in 500hPa accompanied with the warmer air in the early stage. Subsequently, it turned to south airflow ahead of trough accompanied with warm ridge, getting warmer and wetter substantially. Correspondingly, the ground pressure gradually decreased and the convergence increased continuously. In the vertical direction, the inversion layer continued to rise and thicken, the warm advection in the middle and lower layers was obvious, and the vertical shear of the wind was small. In summary, the atmosphere was in a state of extremely static stability, which led to a long and heavy pollution process in North China. During the period of air pollution, the local contribution of PM_2.5 pollution in major cities of Beijing, Tianjin and Hebei region was 40%~60%. The local contribution of PM_2.5 in Beijing was 48%, in which Beijing was mainly affected by the southwest transport channel along the east side of Taihang Mountain from 16 to 17December, then as the result of the decreased wind speed afterwards, local and surrounding cities contributed more.