The PM_2.5 samples were synchronously collected at three sites (Nanjing, Suzhou and Lin'an) in the Yangtze River Delta (YRD) region, China from January 9^th to 31^st to investigate the characteristics and sources of water-soluble ions in atmospheric PM_2.5 in the haze and mist days. Ten water-soluble ions (Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, NO₂^-, F^-, NO₃^- and SO₄^2-) were determined by ion chromatography. The results revealed that the Yangtze River Delta region suffered serious air pollution during winter, with daily average concentrations of PM_2.5 in Nanjing, Suzhou and Lin'an were (150.54±106.63), (159.37±93.77) and (123.64±100.61) μg/m³, which were (2.13±1.42), (2.01±1.25) and (1.65±1.34) times as high as the national standard, respectively. During the observation period, concentrations of PM_2.5 and total water-soluble ions (WSIs) in the mist days were obviously higher than those in the haze days, with the concentration of SO₄^2-, NH₄⁺ and NO₃^-(SNA) increasing significantly, which were 1.70, 1.62 and 1.48 times the concentration of the haze days. This suggests that the elevated RH would largely promote the secondary formation of sulfate and nitrate. Due to higher liquid water content in aerosols in mist days, there in situ pH value were 2.55, 2.44 and 2.5 separately. In addition, the sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were all higher in the mist days. Considering the high correlation between relative humidity and SOR, the formation of sulfate could be explained by heterogeneous reaction, which could be accelerated by higher LWC in mist days. The correlation between NOR and the concentration of NH₄⁺ was also relatively high, by linear fitting the concentration of excess NH₄⁺ and NO₃^-, it might come to the conclusion that the nitrate was most probably formed by the homogeneous gas-phase reaction between ammonia and nitric acid. The weaker acidity of the fog days may be more conducive to the occurrence of this reaction.