In this paper, silica sol was used to seal the hole of anodic oxide film on AZ31 magnesium alloy. The immersion time, calcinating temperature and immersion times were discussed respectively. Scanning electron microscope (SEM) was employed to characterize the surface morphology of the composite film and potentiodynamic polarization curves were used to analyze the corrosion behaviors. The composition of the anodic films that sealed was studied by energy dispersion spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). It has been found that the corrosion resistance of anodic film was improved with sealing by silica sol. When the dipping time is 1.5 min and the calcination temperature is 350 ℃, the corrosion resistance of the film is the best. Times of dipping and calcination has bigger influential on the performance of the film, when the dipping- calcination was 7 times, the film has good corrosion resistance. After sealing, silica sol was adsorbed on inner hole and outside hole of the anodic oxidation film.