The invention provides a method for preparing penipenem, which comprises the following steps: 1) reacting ketone with phosphorus oxychloride to obtain a compound (II) and reacting the compound (II) which is not separated with a compound (V) directly to obtain double-protective group penipenem (VI); and 2) performing the catalytic hydrogenation of the double-protective group penipenem (VI) to obtain penipenem (I). The reaction route is shown below. R may be benzyl or substituted benzyl. R' may be alkyl, aryl or substituted aryl. The method can allow multiple steps of reactions to be finished at one time and therefore reduces reaction period, improves labor production efficiency and reduces equipment investment in industrialization, solvent types and amount, environmental pollution and industrial production cost.