Polymeric compounds are of great interest as corrosion inhibitors in acidic environment due to their inherent stability. Polymeric film is effectively used for the protection of metals partly owing to their capacity to act as a physical barrier between the metal surface and the corrosive environment. In this paper, a cation polyelectrolyte inhibitor (Quaternary Polyethyleneimine, QPEI) was prepared. The quality and inhibition efficiency of QPEI self-adsorbed films were studied by polarization curve and electrochemical impedance spectroscopy, respectively. The compositions and morphologies of the polymer film on the steel surface were examined with the aids of SEM and XPS, respectively. The results showed that QPEI could greatly retard the corrosion of iron in sulfuric acid, which was attributed mainly to the protective QPEI film formed on the surface. Meanwhile, the inhibition of QPEI could be supposed as a mixed process, in which both cathodic and anodic corrosions were simultaneously restrained. Based on the results of XPS and SEM, it could be evidenced that the QPEI can form a protective polymer layer on the metal surface by adsorption.