Mg-Al LDH film was fabricated on anodized magnesium alloy AZ31 by in-situ growth method. The characteristics of the film were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) observation and electrochemical tests. The results showed that the crystallinity of LDH film was increased with reaction time. When the reaction time was more than 12 h, the LDH film had complete crystal layered structure. The anodic oxide film was sealed basically by the growth of LDH nanosheets after 6 h reaction. The corrosion resistance of the films became better with the increasing of reaction time. However, after 12 h reaction, the corrosion resistance of the film decreased. The formation behavior of the Mg-Al LDH film was proposed. That the competitive growth of LDH grains, which contains dissolution and recrystallization, was proposed to explain the change of the growth and corrosion resistance of the Mg-Al LDH film.