Monofluoroacetic acid (FAcOH), was once widely used in baits as a rodenticide in agriculture. For intentional and unintentional misuses, the incidence of FAcOH poisoning has increased in recent years. Organic fluorine rodenticides such as sodium monofluoroacetate and monofluoroacetamide, the analogs of FAcOH, can easily be converted to FAcOH in vivo and cause injury. It is urgent to establish a simple and sensitive analytical method of FAcOH in human plasma and applied to clinical poison analysis. In this paper, an ultra-fast liquid chromatography-tandem mass spectrometry (UFLC–MS/MS) method was developed for determination of FAcOH in patient plasma. We used isotopic labelled FAcOH-13C2, D2 as the internal standard (IS). Plasma samples were simply precipitated with acetonitrile and the supernatant was injected directly for analysis. The chromatographic column was Phenomenex Luna® Silica column (100×2mm, 3μm), FAcOH was eluted by isocratic elution with a mobile phase of acetonitrile-water contains 5mM ammonium formate with 0.2% formic acid. The retention time of FAcOH was 2.31minA good linear response was from 0.25 to 200μg/mL, with a correlation coefficient of r=0.9980. The limit of detection was 0.05μg/mL. The recoveries at three spiking levers were 89.51–93.03% with relative standard deviations ranged between 2.55–3.61%. This new method has been successfully applied to monitor 31 cases of patients suspected with sodium monofluoroacetate or monofluoroacetamide poisoning.