The work aims to improve corrosion resistance of magnesium alloy. Effects of different content of boric acid content on microstructure and corrosion resistance of Dacromet coating on AZ31 magnesium alloy were studied in methods including electrochemical test, salt spray test, X-ray diffractometer (XRD) and scanning electron microscope (SEM). The Dacromet coating on magnesium alloy consisted of Zn, Mg, MgCrO4, ZnO, MgO, Cr2O3 and CrO3. B2O3 appeared on the coating as boric acid was added. The content of MgCrO4 and MgO decreased while the content of Cr2O3 and CrO3 increased (reached the limit value in the coating containing 2% boric acid) as the addition of boric acid increased. Micropore and micro-cracks were present on the surface of the boric acid-free Dacromet coating. Density of the coating containing 2% boric acid could be improved. The coating density was reduced again with the further increase of boric acid. Corrosion current density of the coating with 2% boric acid was 5.068×10-5 A/cm2, 1 order of magnitude lower than that of the boric acid-from coating. Capacitive arc radius and impedance of the coating on electrochemical impedance spectroscopy reached the maximum as 2% boric acid was added, and the corrosion resistance was the best. Boric acid promotes film formation. Adding 2% boric acid can increase the coating density and improve corrosion resistance of the coating.