The invention relates to a preparation method of biapenem as shown in the formula 1. The method comprises the following steps of: using a biapenem intermediate I as a raw material, using a single solvent--water as a reaction solvent and conducting a hydrogenated deprotection reaction in the presence of alkali and a catalyst. The single solvent water is used as a reaction solvent, 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.