Atmospheric PM2.5 and the corresponding gaseous precursors (SO2、NO2) were simultaneously collected at two typical sampling sites in Xiangtan, central south China from December 3, 2013 to January 14, 2014, and then the concentrations of secondary water-soluble inorganic ions (SO2－4、NO－3 and NH+4) in PM2.5 were measured and analyzed by ion chromatography (IC) method. The sources, formation mechanism and influencingfactors of sulfate and nitrate in PM2.5 were discussed by analyzing the oxidation rates of sulfur (SOR) and nitrogen (NOR) at different air quality levels. The results show that, during the sampling time, the mass concentrations of PM2.5 and secondary water-soluble ions in Xiangtan City were 148.34 and 56.19 μg/m3, respectively. The concentrations of SO2－4、NO－3 and NH+4 accounted for 15.26%, 14.06% and 8.57% of PM2.5 concentration, respectively, and the total concentrations of SO2－4、NO－3 and NH+4 accounted for 37.88% of PM2.5 concentration. With increasing of PM2.5 concentration, the concentrations of secondary water-soluble inorganic ions and gaseous precursors (i.e. SO2 and NO2) increased correspondingly. The concentration of SO2－4、NO－3 and NH+4 in PM2.5 at severe pollution level were 1.93, 2.41 and 2.03 times higher than that at good pollution level. SO2－4 and NO－3 in PM2.5 at different air quality levels mostly existed in the form of NH4NO3 and (NH4)2SO4, while other sulfates and nitrates may also exist at both slightly and moderately polluted periods. The average values of SOR and NOR during sampling period at different pollution levels were above 0.1, indicating that sulfate and nitrate were mainly produced by secondary transformation of SO2 and NO2 in the atmosphere. The ratio of NO－3 to SO2－4 in PM2.5 was 0.89, and all thevalues for PM2.5 at different air quality levels were less than 1 (i.e. 0.78, 0.99, 0.82 and 0.97, respectively), indicating that the main source of the water-soluble ions in winter PM2.5 in Xiangtan city was coal combustion.