OBJECTIVE：To de termine unknown impurities in Cefprozil suspension ，and to identify its structure. METHODS ： LC-HR-MS/MS method was used to detect and identify unknown impurities in Cefprozil suspension. The determination was performed on Thermo HyPURITY TM C18 with mobile phase consisted of acetonitrile- 0.013％ formic acid solution （gradient elution ） at the flow rate of 1.0 mL/min. The detection wavelength was set at 230 nm，and column temperature was 40 ℃. The sample size was 20 μL. ESI+ full scanning was carried out with electrospray ion source scanning range was mass-charge ratio （m/z）100-1 500 with spray voltage of 3.8 kV，metal capillary temperature of 320 ℃，sheath gas pressure of 60 Arb，auxiliary gas pressure of 10 Arb，spray temperature of 280 ℃. RESULTS ：Under this condition ，the detection limit of impurity K was 0.202 μg/mL. RSDs of precision and reproducibility tests were both lower than 4％. Three unknown impurities were found around impurity K ，which were isomers of each other. The retention time of ions were 17.83-19.31 min，and the secondary parent ion were all m/z 436.150 0[M+ H]+，which may be the product of ring opening and dehydration of cefpropene. CONCLUSIONS ：Three unknown impurities near impurity K in Cefprozil suspension were detected by this method.