The invention discloses a preparation method of an intermediate for synthesizing carbapenems penicillin etapenem having the formula of VII, and a preparation method and application in the preparation of the carbapenems penicillin etapenem thereof. The preparation method comprises the following steps: allowing a compound having the formula of VI to hydrolyze in acidic condition in dichloromethane or tetrahydrofuran, wherein PMB is shown in figure (1), PNZ is shown in figure (2), and Ac is acetyl. The intermediate can be used for preparing etapenem side chain, and further etapenem. The preparation method can be carried out at the room temperature, with the advantages of mild reaction condition, low cost, simple operation, simple experiment condition, no requirement of expensive reagent, high yield, and easily realized industrial production.