Objective To study the microstructure and film-forming mechanism of chromium-free passivation film. Methods The corrosion resistance property of passivation film was determined by electrochemical tests. The relationship between the smoothness of the passivation film and the corrosion resistant properties was evaluated by SEM analysis. The element and constitution of passivation film were analyzed by XRD, EDS and XPS tests. Results After passivating treatment of the galvanized sheet, the passivation film could restrain the anodic reaction of zinc layer and decrease the corrosion rate. There was no direct relationship between the smoothness of the passivation film and the corrosion resistant properties as shown by SEM analysis. The element and composition of passivation film could not be tested by XRD but could be analyzed by EDS and XPS. The passivation film mainly contained zinc, carbon, oxygen, nitrogen, silicon etc. Conclusions The organic macromolecules formed the main frame in which SiO2 and silicate were absorbed. The unique combination formed a dense passivation film which could restrain the anodic reaction of zinc layer.