Abstract: Ferric salts are typically used as chemical agents to improve the phosphorus removal efficiency in municipal wastewater treatment plants. To investigate the influence of ferric salts on a denitrifying phosphorus removal system, the contaminant removal efficiencies, the formation and transformation of extracellular polymeric substances (EPS) and intracellular storage and measurement of intracellular Fe³⁺ content were studied by dosing different Fe³⁺ concentrations with batch experiments to investigate the influence of Fe³⁺ concentration on the denitrifying phosphorus removal system. The results indicated that:1) when dosing Fe³⁺ was below 10 mg/L, the phosphorous removal efficiencies increased from 88.4% to 100%. However, the phosphorous removal efficiencies decreased slightly when dosing Fe³⁺ concentration was higher than 10 mg/L, and decreased to 84.4% at dosing Fe³⁺ concentration of 25 mg/L. 2) The content of EPS increased with low concentration of Fe³⁺ (10 mg/L) and decreased with high concentration of Fe³⁺ (>10 mg/L) because of the complex reaction between Fe³⁺ and functional groups such as hydroxyl and amino. 3) Fe³⁺ dosing slightly inhibited the variations of PHA and glycogen, and phosphorus release by DPAO (denitrifying phosphorus accumulation organism) in anaerobic phase. However, the utilization of electron acceptor NO₃^--N, the phosphorus adsorption and transformation rates of intracellular PHA and glycogen in anoxic phase were strongly inhibited at high Fe³⁺concentration dosing (>10 mg/L). 4) The intracellular Fe³⁺ concentration increased by 144% at the end of anoxic phase, at Fe³⁺ dosing concentration of 25 mg/L. This indicates that the inhibitory effect of Fe³⁺ on denitrifying phosphorus removal system was mainly because Fe³⁺ was absorbed by the microorganisms accompanied with phosphorus absorption in anoxic phase, influencing the activity of enzyme directly. Moreover, the long-term influence of Fe³⁺ dosing on performance of denitrifying phosphorus removal system should be investigated to optimize the Fe³⁺ dosing in wastewater treatment plants.