NOVELTY - Preparing 7-amino-3-vinyl cephalosporanic acid comprises: (i) adding 4-methoxybenzyl 3-chloromethyl-7-(2-phenylacetamido)-3-cephem-4-carboxylate (GCLE), triphenyl phosphine, sodium iodide, organic solvent, formaldehyde aqueous solution and sodium hydroxide to a reaction kettle, reacting, stirring, performing vacuum distillation to remove dichloromethane to obtain an adhesive material; and (ii) adding phenol and catalyst, reacting, stirring to obtain the adhesive mixture, adding an extractant, adjusting pH, allowing to stand, separating solid crystal in the aqueous phase, and filtering.    USE - The method is useful for preparing 7-amino-3-vinyl cephalosporanic acid.    ADVANTAGE - The method is economical; and consumes less energy; has short reaction time; and produces good quality product.    DETAILED DESCRIPTION - Preparing 7-amino-3-vinyl cephalosporanic acid comprises: (i) adding 4-methoxybenzyl 3-chloromethyl-7-(2-phenylacetamido)-3-cephem-4-carboxylate (GCLE), triphenyl phosphine, sodium iodide, organic solvent, formaldehyde aqueous solution and sodium hydroxide to a reaction kettle, reacting, stirring at 10-35 degrees C, stopping reaction when the pH of the reaction system is less than or equal to 8, and performing vacuum distillation to remove dichloromethane to obtain an adhesive material; and (ii) adding phenol and catalyst to the adhesive material, reacting, stirring at 30-50 degrees C to obtain the adhesive mixture, stopping the reaction when the content of the adhesive material in the adhesive mixture is less than 0.5%, adding an extractant for extraction, adding sodium bicarbonate solution to adjust the pH to 8.5-10.5, allowing to stand the solution in static state for split-phases, maintaining the upper-layer aqueous phase and adding sulfuric acid to adjust the pH to 2.9-3.05, separating the solid crystal in the aqueous phase, and filtering to obtain the solid crystal, which is 7-amino-3-vinyl cephalosporanic acid. The molar ratio of the GCLE, triphenyl phosphorus, sodium iodide, organic solvent, formaldehyde aqueous solution, sodium hydroxide, phenol and extractant is 1:1-1.1:1-1.1:4-10:1-1.1:1-1.05:1-10:1-5. The organic solvent and extractant are methane dichloride. The catalyst is glacial acetic acid, trifluoroacetic acid or oxalic acid, and the amount of the phenol is 10-20%.