A deep yellow-color molybdate conversion coating on LY12 aluminum alloy surface was obtained by immersion process. The experimental results showed that the treating process was simple and the speed of formation of the coatings was fast, and the corrosion resistance of aluminum alloy greatly increased. The treated sample had a good pitting corrosion resistance in 3.5%NaCl solution. SEM experimental results show that the conversion coatings consist of spherical particles differing in size. XPS analysis indicates that the composition of outer layer of the coatings consist of MoO2 and a spot of MoO3.The molybdenum exist in the inner layer of conversion coating in the form of tetravalent status. The mechanisms of coatings formation and corrosion resistance are discussed.