A method for high vacuum diffusion welding of magnesium alloy, which is characterized in that the method includes: first oxidizing the entire magnesium alloy component, and then processing and cleaning the surface of the magnesium alloy component to be welded, so that the magnesium alloy component to be welded is exposed After the fresh metal is assembled, high vacuum diffusion welding is performed; the specific steps of the oxidation treatment include:/nS1. Chemical degreasing: Soak the magnesium alloy parts in a chemical degreasing solution, and then clean them with 60-90°C water. Then rinse with room temperature water; /nS2. Pickling: Soak the magnesium alloy parts in the pickling solution, and then rinse with clean water; /nS3. Chemical oxidation: Put the magnesium alloy parts into the chemical oxidation solution to react, and then Soak in 60-70℃ water for 5-10s, and then dry in an oven at 40-60℃ for 30-60min;/n In step S2, the pickling solution contains 150-200ml/L H3PO4 and 80-100g/L Mixed solution of NH4HF2;/n In step S3, the chemical oxidation solution is a mixture containing 140~160g/L K2Cr2O7, 1~3g/L CrO3, 2~4g/L (NH4)2SO4 and 10-20ml/L HAC liquid; the temperature of the chemical oxidation solution is 60~80°C; the reaction time is 0.5~2min; /n high vacuum diffusion welding process parameters are welding temperature 430°C, welding pressure 10MPa, holding time 3h, vacuum degree 6 ×10-3Pa. /n