From 12 to 17 December 2019, a heavy pollution process with a long duration and extensive influence range of PM2.5 appeared in Hunan Province. This paper comprehensively applies the AQI hourly primary pollutant data and various meteorological observation data released by Hunan environmental monitoring stations. Changsha is selected as the representative station to deeply explore the meteorological conditions, evolution characteristics, and boundary layer characteristics of this heavy pollution weather process and explore the sources of pollutants and the moving path of the external source air mass. The results show that: this process was an inputsuperimposed local compound pollution event. In the pollution accumulation stage, the weak cold air opened up the pollution transmission channel, which was conducive to the pollutants from the north to the south. The pollution outbreak stage was mainly caused by the local static and stable weather superimposed by the upstream external sources. In the pollution removal stage, the ground in Hunan was mainly affected by strong cold air, the wind increased, and the pollutants were rapidly diluted and diffused. The boundary layer height was negatively correlated with AQI. The existence of nearsurface inversion was an essential condition for the pollution to continue and erupt. The backward trajectory shows that the process in the southern Hunan was the result of the heavy pollution weather process and the superposition of external and local sources.