The invention relates to a preparation method for a microarc oxidation/hydrothermal treatment composite film on the surface of a magnesium alloy. The problem that the existing magnesium alloy is poor in corrosion resistance and bioactivity is solved. A microarc oxidation film is prepared on the magnesium alloy by using a microarc oxidation method, and then a bioceramic film is obtained by using hydrothermal treatment. Hydrothermal reaction liquid is composed of sodium hydroxide, calcium acetate and sodium hexametapolyphosphate. In the preparation method, the microarc oxidation is combined with the hydrothermal treatment; calcium and phosphorus are introduced to the surface of the microarc oxidation film by using the hydrothermal treatment; in addition, holes in the ceramic film are blocked by using hydrothermal treatment; therefore, corrosive media are effectively prevented from entering; the corrosion resistance of the composite film is effectively improved; and the corrosion resistance of the composite film to simulated body fluid is greatly improved by approximately 2 orders of magnitude.