OBJECTIVE：To establish a method for the determination of pyrrotinib concentration in plasma ，and apply it in clinic. METHODS ：After precipitated with methanol ，the plasma sample was determined by LC-MS/MS using imatinib as internal standard. The determination was performed on Ultimate AQ-C 18 column with mobile phase consisted of methanol （containing 0.1％ formic acid ）and water （containing 0.1％ formic acid ）（gradient elution ）at the flow rate of 0.4 mL/min. The column temperature was 40 ℃，and the sample size was 5 µL. The ion source was electrospray ionization source ，and the positive ion scanning was carried out in multiple reaction mode. The ion pairs for quantitative analysis were m/z 583.4→138.3（pyrrotinib）and m/z 494.5→ 393.4（internal standard ），respectively. Thirty breast cancer patients taking pyrrotinib were collected from the Affiliated Hospital of Qingdao University during Jun.-Nov. 2020 to determine their steady-state trough concentrations of pyrrotinib after a week of treatment. RESULTS ：The linear range of pyrrotinib were 5-300 ng/mL（r＝0.999 3）. The lower limit of quantification was 5 ng/mL. RSDs of intra-day and inter-day were not higher than 9.30％，and relative errors （REs）ranged －6.70％-5.04％. REs of stability tests were in the range of －1.92％-5.42％. The extraction method ，matrix effect and residual effect did not affect the quantitative analysis of the substance to be tested. The steady-state trough concentrations of pyrrotinib were 32.6-82.8 ng/mL，with an average plasma concentration of 53.8 ng/mL；there was about 2.54 fold individual difference. CONCLUSIONS ：Established LC-MS/MS method is simple ，sensitive and accurate ，and can be used for the plasma concentration monitoring of pyrrotinib in breast cancer patient.