The invention belongs to the technical field of medicine, and mainly relates to a preparation method of a cefotiam hexetil process impurity of 2-(2-(2-(2-(2-(2-aminothiazole-4-yl)-acetamido) thiazole-4-yl)-acetamido) thiazole-4-yl) acetic acid. According to the method, 2-amino-4-thiazole acetic acid and 2-amino-4-thiazole acetyl chloride are used as raw materials to be dissolved into a solvent; microwaves are used for assisting the base catalysis reaction; the pH is regulated to an acid state; filtration and washing are performed to obtain the 2-(2-(2-(2-(2-(2-aminothiazole-4-yl)-acetamido) thiazole-4-yl)-acetamido) thiazole-4-yl) acetic acid, i.e., the cefotiam hexetil process impurity. The reaction equation is shown as the accompanying drawing. Therefore the method has the characteristics that the reaction time is short; the synthesis operation is simple and convenient; the purification operation is easy; the yield is high; the purity is high; the product can be used as a necessary product of cefotiam hexetil quality control or can be applied to the cefotiam hexetil impurity reference substance study, and the like.