A composite silanes-V-Zr passive film was overlayed on hot-dip galvanized steel. Attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectrometer (XPS) and radio frequency glow discharge optical emission spectrometry (rf-GD-OES) were used to characterize the molecular structure of the silanes-V-Zr passive film. The mechanism of film formation was discussed. The results show that the silane molecules are crosslinked as the main film former and inorganic inhibitor is even distributed in the film. The fitting peak of 100.7 eV in XPS single Si2p energy range spectra of the composite silanes-V-Zr passive film and the widening and strengthening of the Si—O infrared absorption peak at 1 100 cm-1 indicate that the silanes were adsorbed on the surface of zinc with chemical bond of Si—O—Zn, and the silane molecules were connected with each other by bond of Si—O—Si. Two characteristic absorption peaks of amide at 1 650 and 1 560 cm-1 appear in the infrared spectroscopy of the composite silanes-V-Zr passive film, and a characteristic absorption peak of epoxy groups at 910 cm-1 disappears in the infrared spectroscopy of the passive film. The results indicate that γ-APT can be prepared through nucleophilic ring-opening of ethylene oxide in γ-GPT molecule to form C—N covalent bonds. The rf-GD-OES results indicate that there is a oxygen enriched layer in 0.3 μm depth of the composite silanes-V-Zr passive film. Moreover, ZrF4, ZrO2 and some inorganic matter obtained by the reaction during the forming process of the composite silanes-V-Zr passive film are distributed evenly throughout the film. According to the film composition, the physical processes and chemical reactions during the film forming process were studied by using ATR-FTIR. Based on this, the film forming mechanism was proposed.