The invention provides a method for preparing a micro-arc oxidized/chemically converted composite coating on the surface of Mg-Li alloy. The method comprises the following steps: in an electrolyte consisting of 3 to 30 grams of silicate per liter, 1 to 10 grams of sodium hydroxide per liter, 10- 30 milliliters of triethanolamine per liter and 1 to 3 liters of distilled water, pretreated Mg-Li alloy is subjected to micro-arc oxidation for 4 minutes by use of a constant-current micro-arc oxidation mode, at the pulse frequency of 2,000 Hz, with the duty cycle of 15 percent and the current density of 5 A/dm<2>; the Mg-Li alloy subjected to the micro-arc oxidation is completely placed in a chemical conversion solution; the conversion solution consists of 5 to 40 grams of molybdate per liter, 1to 10 grams of sodium fluoride per liter and 1 to 15 milliliters of hydrogen peroxide with the concentration of 30 percent; the reaction temperature of chemical conversion is 45 DEG C; the reaction time of chemical conversion is 0.5 to 3 hours; and after the chemical conversion treatment, the film-covered Mg-Li alloy is taken out and dried with cold air. The method has the advantages of high efficiency, low cost, simple process and no pollution on environment, and can improve the corrosion resistance of the Mg-Li alloy and widen the application range of the Mg-Li alloy.