Beijing,Shijiazhuang and Tangshan were selected as the typical cities in the Beijing-Tianjin-Hebei region to investigate the seasonal variation characteristics of secondary water-soluble inorganic ions and compare the pollution characteristics and physicochemical property of the secondary water-soluble ions between heavy pollution period and other periods.Then CAMx-PSAT model was applied to quantitatively analyze the contribution on PM_2.5 and SNA concentration from pollution sources in BTH region during different seasons.Results showed that PM_2.5 concentration in these cities decreased year by year,and the maximum of SO₄^2-,NO₃^- and NH₄⁺ concentration mostly in winter at the same time,illustrating the related correlation of their concentrations.The mass concentrations of SO₄^2-,NO₃^- and NH₄⁺increased significantly during heavy pollution period compared with other periods.The largest concentration ratio of SNA appeared in 1~2 days before heavy pollution days.The formation of heavy pollution was the combined effects of local pollutant emission and external source region transport.The contribution of external sources to NO₃^- was higher than that of SO₄^2- and NH₄⁺.In addition,the concentration of PM_2.5,SO₄^2- and NO₃^- were mostly contributed from traffic sources,resident sources and industrial sources,and the resident sources were the most important contributor for NH₄⁺ concentration.