Accumulating evidence indicates that natural ent-kaurane diterpenoids show great potential for medical treatment of different pathological conditions including cytotoxicity, antibacterial, and anti-inflammatory activity. Among a variety of diterpenoids tested, (-)-pseudoirroratin A displayed a promising antitumor property in vitro and in vivo. However, this diterpenoid could merely be isolated in a limited amount from a rare source of Isodon pseudoirrorata. To overcome such scanty source, we developed a novel, facile, and efficient semisynthetic strategy to prepare (-)-pseudoirroratin A from natural (-)-flexicaulin A, which can be expediently obtained from I. flexicaulis in a great quantity. The three-dimensional structure and the absolute configuration of our synthetic diterpenoid have been determined and confirmed with the X-ray crystallographic analysis. More importantly, we demonstrated for the first time that pseudoirroratin A exerted significant cytotoxicity against human colorectal carcinoma cells via an induction of apoptosis, as well as a remarkable suppression on tumor growth in a colon cancer xenograft mouse model.