OBJECTIVE To establish the de tection method for the adulteration of Bupleurum marginatum in Ganmao qingre granules，and to determine the adulteration limit . METHODS HPLC-MS/MS method was used to detect the content of nepasaikosaponin K in commercial and self -made samples of Ganmao qingre granules ，using electrospray ionization ，and the analysis was carried out under multiple reaction monitoring model in negative mode with m/z 943.6→635.5 as the quantitative ion pair. The calculation method was established for the adulteration proportion of B. marginatum in Ganmao qingre granules . Taking nepasaikosaponin K as the index ，the adulteration limit was determined . RESULTS The linear range of nepasaikosaponin K were 0.051-20.200 μg/mL（r＝0.999 1）. RSDs of precision ，repeatability and stability （24 h）tests were all lower than 2.50%. The average recoveries were 97.58%（RSD＝2.09%，n＝9）. The limit of detection was 0.60 μg/g，the limit of quantitation was 1.80 μg/g. The adulteration ratio of B. marginatum in Ganmao qingre granules had a good linear relationship to the peak area of nepasaikosaponin K in the range of 5%-100%（r＝0.990 9）. The contents of nepasaikosaponin K in 15 batches of Ganmao qingre granules were 2.584-56.661 μg/g. One batch of samples exceeded the proposed adulteration limit （10%），and the unqualified rate was 6.67%. CONCLUSIONS The established analytical method can be used to detect the adulteration of B. marginatum in Ganmao qinggre granules ，the proposed adulteration limit is 10%.