Abstract: Objective: To establish a high performance liquid chromatography-tandem triple quadrupole mass spectrometry method for the detection of human plasma paraquat concentration. Methods: The plasma samples were pretreated with methanol to precipitate plasma protein, and then were separated by a Waters XBridge BEH HILIC column (2.5 μm, 2.1 mm × 100 mm) with acetonitrile-water containing 200 mmol/L of ammonium formate and 0.1% of formic acid as mobile phase and 0.4 mL/min of flow rate. The paraquat was monitored by ESI positive ion mode, multi-reaction ion monitoring (MRM) scanning, and m/z 186.1-171.1 as quantitative transition ion-pair. The plasma paraquat concentrations in patients were determined by the established method, and the clinical values of plasma paraquat concentration and severity index of paraquat poisoning (SIPP) were evaluated by the receiver operating characteristic (ROC) curve. Results: When the plasma paraquat concentration ranged from 50 to 10 000 ng/mL, the linearity was good (R²=0.997), and the lower limit of quantification was 50 ng/mL. The recovery rates and imprecisions of three quality control products at low (100 ng/mL), medium (2 000 ng/mL) and high (8 000 ng/mL) concentration levels all met the requirements, and no matrix effect was found. The pretreated samples were stable at room temperature for 6 hours, and the results were not affected by repeated freezing and thawing for 3 times. The SIPP of 31 poisoned patients was 17.76 (0.30-90.91) h·mg/L. The SIPP in dying patients was significantly higher than that in survival patients (P＜0.05). The area under the ROC curve of SIPP was 0.889, and the optimal cut-off value was 11.679 h·mg/L. Conclusion: The established method is sensitive, accurate, rapid and specific, and suitable for the detection of plasma paraquat concentration in patients.