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The weather condition, atmospheric circulation background, formation mechanism and pollutant trajectory of a heavy air pollution event occurring from 18 to 21 December of 2016 in Qingdao are analyzed based on the data of air quality observation，the routine weather observation and NCEP reanalysis data. The results are as follows: During heavy air pollution process, the 500 hPa circulation was the meridional pattern including two troughs and a ridge at high latitudes, with Qingdao under the control of a weak trough system. When air quality improved, the front and winds at 500 hPa enhanced, and the ground cold front moved to the east. The relative humidity near the ground maintained high constantly during the heavy pollution of haze, and the surface wind speed of less than 26 m〖DK〗·s-1 had no obvious effect on the pollutant diffusion during the heavy pollution. The increasing concentration of pollutants and pollution duration were negatively correlated with such meteorological factor as visibility, wind speed, mixing layer and thickness, and positively correlated with the relative humidity; and the correlation with temperature was low. The 24h inputted pollutant sources in Qingdao were mainly from the northern part of the peninsula, with the main pollutants being PM25.