Lappaconitine is extracted from Aconitum sinomontanum Nakai, which belongs to the Ranunculaceae. Lappaconitine is as a diterpenoid alkaloid used as a nonaddictive analgesic. To assure the rational use of the drug, ultrahigh-pressure liquid chromatography tandem mass spectrometry (UPLC-MS/MS) was conducted to determine lappaconitine in mouse blood and its application to pharmacokinetics. In this study, khasianine was used as internet standard (IS). A UPLC BEH C18 column was used for chromatographic separation and the mobile phase consisted of acetonitrile and 10 mmol/L ammonium acetate (0.1% formic acid). The flow rate of was 0.4 mL/min. Quantitative detection was performed in a multiple reaction monitoring (MRM) mode using an electrospray ionization source in positive mode. Twenty-four mice were randomly divided into four groups, three of which received 2, 4, and 8 mg/kg lappaconitine by intragastric administration, while the other group received 1 mg/kg lappaconitine by intravenous administration. After 0.0833, 0.5, 1, 1.5, 2, 3, 4, and 8 h, blood samples were collected and acetonitrile was used for protein precipitation. A linear calibration relationship (R2 = 0.9979) in the range of 0.1-500 ng/mL in mouse blood indicated good results. The lower limit of quantitation was 0.1 ng/mL and the limit of detection was 0.04 ng/mL. The intra-day and inter-day precision were below 13% and 14%, respectively. The accuracy was 90.1-107.2%, and the recovery exceeded 81.1%. The matrix effect ranged between 102.1 and 108.8%. The absolute bioavailability of lappaconitine was 2.0%. UPLC-MS/MS achieved high sensitivity, speed, and selectivity. Methodological verification indicated this method as suitable for determination of lappaconitine in mouse blood.