Based on the online observation of PM2.5 mass concentration, its water-soluble inorganic ions, and their gaseous precursors during August of 2013 to March of 2014 at the atmospheric supersite in the Pearl River Delta (PRD) region, the inter-action of the secondary compositions and their precursors was discussed, and the pollution properties of the secondary inorganic ions were revealed. During the whole measurement period, the average concentrations of SO42–, NO3– and NH4+ were 16.6 µg m–3, 9.0 µg m–3 and 10.2 µg m–3, respectively, with total contribution to PM2.5 of 55.8%, indicating the significant role of secondary transformation in PM2.5 pollution. The seasonal average total contributions of SO42–, NO3– and NH4+ to PM2.5 varied from 46.0% to 64.3%, lowest in summer and highest in winter. The contributions of SO42– and NH4+ to PM2.5 were relatively stable; while those of NO3– in different seasons were distinct, even dominating PM2.5 in some pollution cases in winter. NH3 was abundant with an annual average concentration of 15.2 µg m–3, facilitating the neutralization of H2SO4 and HNO3 with the average [NH4+]/(2[SO42–] + [NO3–]) equivalent charge ratio of 1.1. The maximum daily peak concentration of HNO3 was as high as 18.6 µg m–3, providing an evidence for the strong oxidizing property of the atmosphere in the PRD region. The theoretical equilibrium constant (Ke) of NH4NO3 is always lower than the observed concentration product (Km = [NH3] × [HNO3]) in spring and winter with higher HNO3 concentrations; while in over 60% of the time during summer and autumn, mainly during daytime, Ke was higher. In general, the strong oxidizing property and NH3 played important roles in the fine particle pollution in the PRD region.