The invention provides a by-product shown in a formula II generated in a synthesis process of tebipenem pivoxil, and a method for preparing the high-purity by-product. The method comprises the steps of: reacting the tebipenem pivoxil with paraformaldehyde in an organic solvent under the catalysis of organic alkali to obtain the product of which the purity is up to over 99% by column chromatography purification, detecting the product by a liquid-phase mass spectrum combined chromatograph, comparing with the tebipenem pivoxil synthetized according to a literature method, and determining the corresponding impurities generated in the synthesis process of the tebipenem pivoxil. Therefore, the by-product shown in the formula II prepared by the method disclosed by the invention can be used as a a reference substance for the research of related matters in a mass research process of the tebipenem pivoxil so as to control the content of the related matters of the material drug tebipenem pivoxil and obtain high-quality tebipenem pivoxil material drug.