AM60B magnesium alloys were processed in sodium silicate based electrolytes by micro-arc oxidation (MAO), and the influence of the concentration ratio of potassium fluoride to sodium silicate (in short as fluoride-silicate ratio) on the microstructure and corrosion resistance of the coatings was quantitatively analyzed with changing the concentrations of KF and Na2SiO3 in the electrolyte. The results show that the main salt Na2SiO3 is essential in the film-forming reaction. When fluoride-silicate ratio was greater than zero, the film-forming reaction was intensified with the increasing of fluoride-silicate ratio, which caused the coatings owning more surface porosity, more number of large pore (> 3μm ), and larger thickness. And newly formed MgF2 and more Mg2SiO4 in the coatings because of the existence of the F- in the electrolyte will contribute to the improvement of the coatings’ anti-corrosion ability. In addition, the competitive and synergistic effect of F- and SiO32- in the electrolyte will vary with the changing of fluoride-silicate ratio, thus affecting the microstructure, phase content, and even the corrosion resistance of the coatings. In this paper, the best fluoride-silicate ratio was 0.5(KF: Na2SiO3 = 7.5:15), since the resulting coating has shown a larger thickness, better density and fewer defects due to the good synergism between F- and SiO32-. That is to say, both large thickness and excellent corrosion resistance of the coating has been obtained at this point.