In this study, we systematically investigated the oxidative degradation of 4-chlorophenol (4-CP) using nano zero valent iron (nZVI, Fe-0) cooperated with citric acid (CA), and the role of CA as well as the mechanism of oxidation were deeply evaluated. 56.2% of 4-CP was removed after 60 min under dark condition without any ventilation of oxygen in the presence of Fe0 and CA. The addition of CA accelerated the release of ferrous ions and promoted the Fenton reaction to generate much more reactive oxygen species (ROS), which was confirmed by EPR. The MINEQL+ 4.6 soft was utilized to calculated the existent species in Fe-0-CA system, and Fe(II)[Cit](-) was found to be the key species playing an important role on the degradation of 4-CP. Fe(II)[Cit](-) adsorbed on iron surface accelerated the transportation of electron from Fe-0 to O-2 to produce more % OH. This study puts forward the possible mechanism of generating ROS on Fe-0 enhanced by CA and provides an application for the oxidation of pollutants.