The invention relates to an improved method for preparing d-biotin from bisbenzyl biotin by debenzylation, which has the advantages of high product quality, low cost and high environment friendliness. The method comprises the following steps: putting bisbenzyl biotin 5-[(3aS,4S,6aR)]-1,3-dibenzyl-2-oxohexahydro-1H-thieno[3,4-d]imidazolyl-4-yl]valeric acid in a reaction kettle; adding hydrobromic acid, stirring and heating to reflux, and reacting under reflux conditions until no raw material dots by TLC (thin layer chromatography) detection; adding an aromatic solvent for extraction, and separating an organic phase from a water phase; concentrating the water phase under reduced pressure to a dry state; adding pure water, stirring, standing to crystallize, and filtering to obtain a d-biotin crystal; concentrating the filtrate, standing to crystallize, filtering to obtain a diamido substance 5-[(2S,3S,4R)-3,4-diamidotetrahydrothienyl-2-yl]valeric acid hydrobromide, adding the diamido substance and an inorganic alkali solution into a reaction kettle, and adding phosgene to carry out cyclization reaction; after the cyclization reaction finishes, regulating the pH value to acidity with an inorganic acid solution, and standing to crystallize; and filtering to obtain the d-biotin 5-[(3aS,4S,6aR)]-2-oxohexahydro-1H-thieno[3,4-d]imidazolyl-4-yl]valeric acid. The method is used for producing d-biotin from bisbenzyl biotin.