A new type of Mg/Al-CO32--layered double hydroxide（LDH） inorganic film was designed and prepared on Q235 steel in situ by the hydrothermal method with urea hydrolysis for the sake of enhancing the corrosion resistance of steel matrix especially in concrete environment. X-ray diffractometer（XRD）， attenuated total reflection Fourier infrared spectroscopy（ATR-FTIR） and scanning electron microscope（SEM） were used to observe the chemical composition and structure characteristics of the LDH film. Electrochemical impedance spectroscopy and linear polarization resistance were measured to evaluate the anticorrosive performance of LDH film. The results show that the LDH film prepared under low metal ion concentration has lower crystallinity and larger size of LDH sheets with relatively lower thickness， while high metal ion concentration will generate magnesium carbonate that would limit the formation of nucleation sites for the further growth of LDH sheets. Therefore， LDH film prepared under the synthesis condition of c（Mg²⁺） = 0.050 0 mol/L， c（Al³⁺） = 0.025 0 mol/L and c（urea） = 0.375 0 mol/L has a complete crystalline layered structure and uniform coverage on the surface of the Q235 steel substrate， which has a positive effect on the corrosion resistance of steel substrate.