Extensive phytochemical investigation on the whole herbs of Delphinium anthriscifolium var. majus led to the identification of fourteen diterpenoid alkaloids, including three new C20-diterpenoid alkaloids (anthriscifolsines A-C, 1-3), six new C19-diterpenoid alkaloids (anthriscifolrines A-F, 4-9), and five know compounds (10-14). Among them, anthriscifolsine A represents a novel C20-diterpenoid alkaloid characterized by a seco C-ring. The structures of the isolated compounds were elucidated by extensive spectroscopic methods, including HR-ESI-MS, X-ray, and 1D and 2D NMR experiments. Bioactivity of compounds 3-6 was evaluated for their cytotoxicity against the MCF-7, HepG2 and H460 cancer cell lines.