A simultaneous method was developed for the analysis of 28 free amino acids using ultrahigh performance liquid chromatography-quadrupole/orbitrap high resolution mass spectrometry with pre-column derivatization,and was used to detect the free amino acids in cruciferous plants.The samples were extracted using ultra pure water,and derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate(AQC) to enhance chromatographic retention.A Waters BEH C18 column was used as stationary phase.Ammonium acetate solution(pH 5.0) and 80% acetonitrile in water were applied as mobile phases for gradient elution.MS detector with electrospray ion source worked in positive ion mode.The results showed that there were more than 25 kinds of free amino acids in cruciferous plants,including 8 essential amino acids.All calibration curves of the analytes exhibited good linearity.The average recoveries of this method ranged from 80.5% to 104.4% with the relative standard deviations of 0.6%-4.4%.The sensitivities of amino acids were different,and the quantitation limit was between 0.01 μmol/L and 1.45 μmol/L.With the advantages of little impurity interference,rapidness and high sensitivity,the method is suitable for the simultaneous detection of free amino acids in plant samples.