A C19 diterpenoid alkaloid represented by where R1 is hydrogen, methyl or ethyl; R2 is hydroxyl, methoxy or ethoxy; R3 is hydroxyl or methoxy; and R4 is hydroxyl or methoxy. The C19 diterpenoid alkaloid is prepared from a plant material belonging to the genus Aconitum of the family Ranunculaceae through extraction, concentration and separation using silica gel column chromatography. An application of the C19 diterpenoid alkaloid in the preparation of cardiotonic drugs and drugs for preventing and/or treating heart failure is also provided.