The invention provides a Doripenem side-chain compound shown by the formula (IX), wherein R represents hydrogen, p-nitrobenzyloxycarbonyl or p-methoxyl carbobenzoxy. The compound does not contain a protective group or only contains one protective group, and can be directly connected with a mother nucleus compound to prepare Doripenem, so that the utilization efficiency of a mother nucleus is improved, the operation is simplified, the hydrogenation reaction efficiency is improved, the manufacture cost of a product is reduced, in addition, the side-chain compound is a crystal so as to have better material attributes including stability, separability or purity, the crystal is reacted with the mother nucleus compound to obtain the Doripenem with high quality, and the advantages of convenience in purification, low cost and convenience in direct use are provided. In addition, the invention provides a preparation method of the Doripenem side-chain compound, which has the advantages of simpleness in operation, and the like, and is suitable to be applied in factories in a large scale. The invention also provides application of the Doripenem side-chain compound in the preparation of Doripenem.