Preparation method of ertapenem and its sodium salt

The invention relates to a preparation method of ertapenem as shown in the formula 1 and its sodium salt. According to the method, water is used as a reaction solvent, an ertapenem intermediate as shown in the formula 2a undergoes a hydrogenated deprotection reaction in the presence of alkali and a catalyst, so as to generate ertapenem as shown in the formula 1 or its sodium salt. The single solvent water is used as a reaction solvent in the method provided by the invention, thus solving the problem of dissolving the catalyst by the reaction solvent, simplifying post-treatment operational step, reducing product degradation and raising product purity. In addition, the preparation method is economical, safe and environmentally friendly, and is more suitable for industrial operation at large scale.