OBJECTIVE：To compare the chemical components in Sinapis alba before and after stir-frying. METHODS ： UPLC-Q-Exactive Obitrap MS was adopted to analyze chemical constituents of S. alba before and after stir-frying. The determination was performed on Waters CORTECS T 3 column with mobile phase consisted of methanol- 0.1％ formic acid solution （gradient elution ）at the flow rate of 0.25 mL/min. The column temperature was 30 ℃ and the sample size was 2 μL. High resolution MS adopted heating electrospray electron source ，positive ion scanning mode ，scanning range m/z 120-1 000. The chemical constituents of S. alba before and after stir-frying were identified by Compound Discover 3.2 software combined with relevant database ，and the content changes of chemical constituents were analyzed by using peak area. Chemometrics analysis was performed for the content changes of chemical constituents using peak area as variable. RESULTS ：A total of 54 chemical components were identified in S. alba ，mainly fatty acids （represented by erucic acid ），alkaloids（represented by sinapine ）， flavonoids. After stir-frying ，the contents of 19 chemical components changed significantly ，of which the contents of 10 components decreased significantly and those of 9 components increased significantly （P＜0.05）. Principal component analysis and orthogonal partial least squares discriminant analysis could clearly distinguish S. alba from stir-fried S. alba . CONCLUSIONS ：The contents of some chemical components of S. alba change significantly after stir-frying ，which may be one of the important reasons for the change of efficacy after stir-frying.