Abstract: Based on the hourly data provided by Changshu Environmental Monitoring Station and chemical composition data of PM_2.5 and lidar monitoring data from November 21st to December 5th in 2018, the pollution characteristics of PM_2.5 and its main chemical composition (water-soluble ions) in a heavy pollution process in Changshu were systematically analyzed. The results showed that Changshu occurred a continuous heavy pollution process from November 24th to December 3rd, the high PM_2.5 concentration mainly occured in higher humidity, lower wind and lower boundary layer weather conditions, and the PM_2.5 concentration was positively correlated with humidity. Accumulation of local pollution under adverse diffusion condition, the increase of moisture absorption of particulate matter under high humidity conditions and secondary transformation were the main causes of pollution. During the observation period, secondary ions(NO₃^-、NH₄⁺、SO₄^2-)accounted for a higher proportion (as high as 97%) of water-soluble ions, especially in the pollution period, which was greatly affected by secondary formation, and NO₃^- accounted for the highest proportion of water-soluble ions. During the whole analysis period, the mean values of SOR and NOR were 0.38 and 0.22, and the values increased significantly during the pollution period, reaching 0.47 and 0.32, respectively. The cumulative growth rates of NO₃^-, SO₄^2- were higher than those of CO only in the pollution period, and the concentration of NO₃^- was higher than SO₄^2-, which indicated the pollution was mainly affected by the secondary generation of NO₂. It was necessary to strengthen the control of industrial emission sources and motor vehicle exhaust sources during heavy pollution period.