To investigate the characteristics and sources of haze pollution and the formation mechanism of sulfate the northern suburb of Nanjing, PM_2.5 samples were collected in March 2016, and the concentrations of water-soluble ions (Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, NO₃^- and SO₄^2-) and carbonaceous components (OE and EC) were measured. The average concentration of PM_2.5 during the sampling period was (103.22±48.5)µg/m³. In contrast to clean days, the formation of secondary sulphate in the polluted days was much stronger related to the oxidation of SO₂ by NO₂ than the oxidation with O₃. The mineral dust could serve as buffered agent and kept the aerosol in weakly alkaline state during polluted days, which was preferred for the secondary sulfate formation. In this work, the secondary organic carbon (SOC) in PM_2.5 was mainly formed from the photooxidation of hydrocarbons with O₃. The main sources of PM_2.5 components in polluted days were motor vehicle emissions, followed by biomass and coal combustion. In clean days, the bulk components were mainly contributed by coal combustion and dust emission with little influences from motor vehicle exhaust.