Method for synthesizing moguisteine active metabolite

The invention relates to a method for synthesizing an active metabolite (+/-)-3-(2-carboxyacetyl)-2-(2-methoxyphenoxy) thiaxolidine of a non-narcotic cough relieving medicament moguisteine. The method is characterized by comprising the following steps of: heating moguisteine serving as an initial raw material and an acid solution or an alkali solution together, alkalifying or acidifying, then extracting the solution by using an organic solvent, evaporating the solvent under reduced pressure, and performing re-crystallization by using an alcoholic solvent, thus obtaining a high-purity target product. By adopting the method, the moguisteine in-vivo active metabolite with a cough relieving effect can be prepared in vitro under mild conditions.