The micro-arc oxidation (MAO) ceramic coating on AZ31 magnesium alloy was treated by hydrothermal treatment in NaOH solution to study the effect of the concentration of NaOH solution on the microstructure and corrosion resistance of the obtained Mg(OH)₂/MAO composite coating, the formation mechanism and the corrosion mechanism of the hydrothermal film was annalysed at the same time. The results showed that MgO in the surface of MAO ceramic layer was partially dissolved during hydrothermal reaction. And the released Mg ²⁺ was combined with OH ^- in hydrothermal solution to form Mg(OH)₂ nanosheets which deposited on the surface of ceramic layer and inner wall of the pores. With increasing of NaOH concentration of the hydrothermal solution, more Mg(OH)₂ formed during hydrothermal reaction and sealed the inherent defects such as pores and cracks in the MAO ceramic layer, which improved the compactness of the coating. In addition, the electrochemical test results demonstrated that the Mg(OH)₂/MAO composite coating prepared by MAO and hydrothermal treatment achieved better corrosion resistance than the single MAO ceramic layer, and the corrosion resistance of Mg(OH)₂/MAO composite coating increased with the increasing concentration of the hydrothermal solution. And the immersion test showed that Mg(OH)₂/MAO composite coating could provide long-term corrosion protection for magnesium alloy.