To establish a method for the determination of the related substance of amoxicillin capsules, and investigate the difference between the new method and Chinese pharmacopoeia.The related substances were determined by HPLC.The mobile phase A was 2.72 g/L potassium dihydrogen phosphate, and the p H was adjusted to 5.0 by 1 mol/L potassium hydroxide.The mobile phase B was methanol.The detection wavelength was 210 nm, the column temperature was 40 °C, the volume flow rate was 1.5 m L/min, and the sample size was 10 μL.The new method can sep. impurities more effectively with good precision, and the sample solution has no adsorption to the filter membrane.The recoveries of impurities are 92% ∼105%, and the linear relationship between the 30%, 50%, 80%, 100%, 120% of the control content and the impurity limit concentration is good.The new method can sep. impurities more effectively, and it is accurate and reliable.It can be used for the determination of amoxicillin capsules.