The microstructure and corrosion resistance of ceramic coatings formed by micro-arc oxidation on magnesium alloys in electrolyte systems with different Na2B4O7 concentrations was investigated by scanning electron microscopy and electrochemical workstation respectively. The function of sodium tetraborate in the formation and growth process of micro-arc oxidation coatings was analyzed. The results showed that a compound of (B4O7) 2- and Mg2 on the anode surface was formed the water-insoluble material of Mg(BO2)2 before the occurrence of micro-arc discharge. As the concentrations of Na2B4O7 increased from 3g/L to 15g/L, the morphology of deposited layer transited from granular to net structure gradually. The formation of Mg(BO2)2 layer with a high impedance characteristic set up a suitable electric field condition for MAO process. After micro-arc discharge happened, Mg(BO2)2 on the anode surface by raising the discharge voltage enhanced the strength of single pulse discharge under constant pulse width and peak current, and then speed up the growth of micro-arc oxidation coatings.