Abstract: The air quality data published by the environmental authorities, the meteorological data collected at Hefei observatory during 2013-2015, together with the vertical extinction coefficient data observed by lidar and water-soluble inorganic ions data of aerosols obtained during scientific field experiments were used to analyze characteristics of severe PM_2.5 pollution (daily average PM_2.5 concentration >150 μg/m³) in Hefei. The results show that:(1) the PM_2.5 pollution showed evident spatial differences with more severe PM_2.5 pollution days at sites in northeastern Hefei and fewer pollution days at sites in southwestern Hefei. Monthly variations of severe PM_2.5 pollution days were similar at all sites with the largest difference occurring in January. The PM_2.5 concentration showed an evident diurnal variation with two maxima in the morning and evening respectively, and the morning maximum in severe pollution days often occurred later than in light polluted days; (2) in severe PM_2.5 pollution days, the concentrations of other gas pollutants except O₃ increased obviously; (3) severe PM_2.5 pollution days usually corresponded to haze and light fog, accompanied with light winds and stable stratification. The vertical extinction coefficients of aerosols below 600 m at the noon in severe pollution days were much higher than those in other days, while the occurrence height of maximum extinction coefficient decreased; (4) in severe pollution days, the water-soluble inorganic ions in PM_2.5 became more abundant, and the percentage of NO₃^- increased most, exceeding the percentage of SO₄^2-. The above results advance our understanding of the role of nitrate in the formation of high level PM_2.5 and are helpful for the forecast and control of PM_2.5 pollution.