A total of 131 samples of PM2.5 were collected at urban area of Chengdu from April, 2009 to January, 2010. The secondary water-soluble inorganic ions (NH4(+), NO3(-) and SO4(2-)) in PM2.5 were determined to study the pollution characteristics by Ion Chromatography method. The results showed the NH4(+), NO3(-) and SO4(2-) concentrations were (10.4 ± 8.6), (19.7 ± 14.6) and (32.8 ± 21.8) μg x m(-3), accounting for (5.5 ± 2.8)%, (11.1 ± 3.5)% and (19.3 ± 6.4)% of PM2.5, respectively, and the sum of these three ions accounted for (35.9 ± 12.7)%. Meanwhile, the seasonal characteristics of secondary inorganic ions were obvious. The seasonal concentrations order of secondary inorganic components was SO4(2-) > NO3(-) > NH4(+) in summer and winter, and the proportion of secondary inorganic ions of PM2.5 was 39.4% in summer and 44.3% in winter. The results of Correlation Analysis showed that most of NH4(+), NO3(-) and SO4(2-) existed as NH4HSO4, (NH4 )2SO4 and NH4NO5 in Chengdu. And the average ratio of NO3(-)/SO4(2-) indicated that the main source of sulfur and nitrogen in the atmosphere was given priority to a fixed source. In addition, the sulfate oxidizing rate (SOR) and nitrate oxidizing rate (NOR) were 0.33 ± 0.12 and 0.19 ± 0.09, respectively. It showed that SO4(2-) and NO3(-) in PM2.5 were due to the secondary transformation.