HPLC method was used for the determination of related substances in (E)-4-[2-(4-chlorophenoxy)-2-methylpanoyloxy]-3-methoxyphenyl acrylic acid. The separation was achieved by Ultimate XB-C18 column (4.6 mm×150 mm,5 μm) with mobile phase composed of methanol-1% acetic acid water (70∶30) at a wavelength of 275 nm. The results showed that (E)-4-[2-(4-chlorophenoxy)-2-methylpanoyloxy]-3-methoxyphenyl acrylic acid with various intermediates and compulsory destruction of degradation products were well separated. The impurity limit in three batches of API was less than 0.1% and the main impurity C was isolated and identified. Within the range from 0.20 to 59.96 μg/mL, the mass concentration of impurity C has good linear relationship with the peak area (r=0.999 9). The control method of related substances for (E)-4-[2-(4-chlorophenoxy)-2-methylpanoyloxy]-3-methoxyphenyl acrylic acid was established by impurity reference method and self-high and low concentration comparison. Methodological validation can be used for the detection of related substances of (E)-4-[2-(4-chlorophenoxy)-2-methylpanoyloxy]-3-methoxyphenyl acrylic acid.