The invention discloses a preparation and application method for conversion liquid of a corrosion resistant oxide film on magnesium alloy surface, which is characterized in that: the conversion liquid adopts rare-earth-containing salt, permanganate, persulphate, nitrate and perchlorate as composite oxidant, zirconium salt, vanadic salt and strontium salt as film forming promoter, boric acid, aminoacetic acid-hydrochloric acid solution as pH buffer regulator, pyridine, thiourea and derivatives, tannins, phytic acid and the salt compound as corrosion inhibitor, sodium dodecyl benzene sulfonate and dodecyl phenol polyoxyethlene ether (OP-10) as wetting agent; the conversion liquid is free from hexavalent chromium and environment-friendly; heating is not required for chemical conversion treatment, and the treatment time is 1 to 5min; composite oxide film with good corrosion resistance performance comprising rare-earth composite oxide, magnesia and manganese oxide can be rapidly prepared at on magnesium alloy surface at ambient temperature. The preparation method for conversion liquid of a corrosion resistant oxide film on magnesium alloy surface has the advantages of fast film forming, simple process, uniform film, strong corrosion resistance and less environmental pollution.