OBJECTIVE： To establish a method for determination the genotoxicity impurities （methyl methanesulfonate， ethyl methanesulfonate and isopropyl methanesulfonate） in mesylate nafamostat raw materia. METHODS： GC-MS was conducted， and the genotoxicity impurities were extracted by dichloromethane. The column was DB-5 capillary column by programmed temperature， the inlet temperature was 240 ℃， column flow was 3.0 ml/min， purge flow was 6.0 ml/min， sample mode splitless injection， carrier gas was high purity helium， detector is a mass spectrometer detector， ion source temperature was 230 ℃， the interface temperature was 230 ℃， the delay time of solvent was 2.5 min， ionization mode was electron impact， detector voltage was respect to the tuning results， scanning （detection） method was selective ion monitoring， electron energy was 70 eV， and the injection volume was 1.0 μl. RESULTS： The separation degree of 3 impurities were greater than 2.0； the linear range of 3 impurities were 0.10-20 μg/ml（r≥0.999 5）； RSDs of precision， stability and reproducibility tests were lower than 2％； recoveries were 97.7％-104.8％ （RSD＝2.8％，n＝9），102.5％-110.7％（RSD＝2.6％，n＝9） and 103.0％-107.6％（RSD＝1.6％，n＝9）. CONCLUSIONS： The method is simple， accurate， sensitive and rapid， and can be used for the genotoxicity impurities in mesylate nafamostat raw materia.