［Objective］To improve the corrosion resistance of magnesium alloys.［Methods］A composite coating consisting of a magnesium hydroxide bottom layer and a calcium silicate（CaSiO .） top layer was constructed on AZ91D magnesium alloy using a one-step hydrothermal method. Theinfluence of reaction conditions on the structure and performance of the coating was systematicallyinvestigated. The optimal preparation parameters were determined by regulating the hydrothermalreaction temperature and time. Coating performance was comprehensively evaluated using polarizationcurves，electrochemical impedance spectroscopy（EIS），and immersion weight loss tests.［Results］The prepared coatings significantly enhanced the corrosion resistance of the substrate under all investigatedreaction conditions. The corrosion current density of the best-performing sample was approximatelyfour orders of magnitude lower than that of the uncoated alloy. Furthermore，the coating maintained a stable structure during prolonged exposure to a corrosive environment，demonstrating excellent corrosion resistance.［Conclusion］This hydrothermal coating strategy provides a green，efficient，and feasible approach for surface protection of magnesium alloys.