The invention belongs to the technical field of medicine synthesis, and especially discloses a method for preparing 7-amino-3-vinyl cephalosporanic acid. The preparation method takes GCLE as an initial raw material, the GCLE and sodium iodide and triphenylphosphine are reacted in a mixing system of an organic solvent and water to generate phosphine salt, a certain amount of alkali lye is added in an organic phase for reacting to generate the corresponding phosphorus ylide, excess free alkali in an organic phase is removed through washing, formaldehyde and ylide are added for a wittig reaction, the organic phase is concentrated to a certain weight, phenol is added for removing carboxyl protection, amino protection is removed, crystallization is carried out to obtain 7-AVCA. The synthesis method does not require a mixing organic solvent, can realize recovery of sodium iodide and formaldehyde, has the advantages of high product yield, good quality, no three wastes generation, and environmental protection, and is suitable for large scale industrial production.