To establish an assay of determination of lactic acid in milrinone injection, HPLC was carried out on a ODS-C18column (4.6 mm × 250 mm, 5 μm) column, the mobile phase A consisted of 0.05 mol/L potassium dihydrogen phosphate solution (with phosphoric acid to adjust the pH to 3.0), the mobile phase B consisted of dipotassium hydrogen phosphate solution (taking dipotassium hydrogen 2.7 g, adding water 800 mL dissolved and triethylamine 2.4 mL, with phosphoric acid to adjust the pH to 7.5)-acetonitrile (70:30), gradient elution, 0-7 min, 7-8 min, 8-15 min, 15-16 min, 16-27 min of the mobile phase were as follows: mobile phase A: 100%, mobile phase B: 0→100%, mobile phase B: 100%, mobile phase A: 0→100%, mobile phase A: 100%, with the flow rate of 1.0 mL/min, the detection wavelength was 210 nm.The results showed that the standard curve was in good linearity in the range of 198.16-1387.12 μg/mL, r = 0.9998 for determination of its related substances.The average recovery was 100.42%, RSD was 0.89%.The method is accurate, reliable, reproducible and can be used for quality control of lactic acid in milrinone injection.