Micro-arc oxidation was conducted on AZ31 Mg alloy under CC mode by single variable experiments to improve its corrosion resistance and bio-activity. The microstructure, Ca/P ratio and corrosion rate were studied to investigate the influence of the Ca- or P-containing compositions on the coating. Furthermore, the properties of the optimized coating were studied. The results reveal that Ca/P ratio depends on the concentration of Ca(Ac)2, (Na2PO3)6 and NaH2PO4. With the increase of Ca(Ac)2, the coating thickness decreases, but it becomes more uniform; while (Na2PO3)6 has little effects on surface morphology or coating thickness. The content of P increases with (Na2PO3)6. The more NaH2PO4, the bigger the pore size and the smaller Ca/P ratio. The optimized coating possesses a flat and compact surface covered by micropores with diameters of about 10.7 μm and there are microcracks. And the coating phases are mainly MgO, Mg and a small amount of Ca2P2O7 and SiO2, which make the coating harder and more resistant to corrosion. The coating roughness Ra is 0.4482 μm and the wetting angle is 42.65°, which are good for cells to adhere to and grow on.