To obtain diterpene glycosides from an aqueous extract of the aerial parts of Isodon henryi and further investigate their cytotoxicities, in this study, a total of seven compounds were isolated, including six ent-kaurane diterpene glycosides (1–6) and one diterpene aglycon (7). Among the seven ent-kaurane diterpenes obtained, four were novel compounds, including ent-7,20-epoxy- kaur-16-en-1α,6β,7β,15β-tetrahydroxyl-11-O-β-d-glucopyranoside (1), ent-7,20-epoxy-kaur-16-en- 6β,7β,14β,15β-tetrahydroxyl-1-O-β-d-glucopyranoside (2), ent-7,20-epoxy-kaur-16-en-6β,7β,15β- trihydroxyl-1-O-β-d-glucopyranoside (3), and ent-7,20-epoxy-kaur-16-en-7β,11β,14α,15β-tetrahydr- oxyl-6-O-β-d-glucopyranoside (4), and three were isolated from this plant for the first time (5–7). Their structures were elucidated by utilizing spectroscopic methods and electronic circular dichroism analyses. Furthermore, the cytotoxicities of all seven compounds were investigated in four human cancer cell lines, including A2780, BGC-823, HCT-116, and HepG2. The IC50 values of these diterpenes ranged from 0.18 to 2.44 mM in the tested cell lines. In addition, the structure–cytotoxicity relationship of diterpene glycosides was also evaluated to study the effect of glycosylation on the cytotoxicity of diterpene compounds.