The invention discloses an intermediate shown as the formula II for synthesizing carbapenem penicillin ertapenem, and a preparation method thereof. The preparation method comprises the following steps: reacting a compound shown as a formular I with 4-methoxybenzyl chloride in the presence of an organic alkali reagent as a catalyst in a nonpolar solvent. The invention also discloses the application of the intermediate on the preparation of the carbapenem penicillin ertapenem. The PMB is the formula III. The intermediate compound can be prepared into ertapenem side chain and then further into ertapenem. The preparation method has the advantages that the conditions of each reaction are mild, the reaction can be carried out at the room temperature, the cost is low, the method is easily applied on an industrialized production, the operation is simple, the experimental condition is simple, no expensive reagent is required, the yield is high and reaches above 90%, and the purity is high.