The densification process of lanthanum and cerium conversion film on AZ91 magnesium alloy was achieved by ammonium phosphate, and the orthogonal expemenrit was used to optimize the densification process. The properties of the film were investigated by scanning electronic microscopy (SEM), X-ray diffractometer (XRD) and electrochemical methods. The results reveal that the treatment temperature was 50 ℃, (NH₄)₃PO₄ mass concentration was 5%, densification of time was 4 min under the optimal treatment techniques. The SEM results show that after the densification of the rare earth conversion film, cracks in the films decreased significantly, and the denseness of the conversion coatings was improved. Results of XRD analysis indicated that the dual rare-earth conversion film with densification process are composed of rare earth oxides and hydrated hydroxides and phosphates and magnesium oxide . The electrochemical results show that the corrosion resistance of the conversion film is strengthened. High temperature oxidation test show that Lanthanum and Cerium conversion film with densification process is better than the film without densification process.