Seventeen 13,28-epoxy triterpenoid saponins obtained from Ardisia gigantifolia STAPF. were evaluated their anti-proliferative activities on MCF-7 cells. The structure–activity relationship analysis indicated that CH₃ group at C-30, four saccharide units with L-rhamnose at R6 in the sugar units are crucial for the cytotoxic activity on MCF-7. Compounds 1 , 2 , 6 , 7 , 12 , and 14 were selected to identify the anti-proliferative activity on the other three breast cancer cell lines (T47D, MDA-MB-231 and SK-BR-3). Compounds 2 , 6 , and 7 with good activity on MCF-7 also showed activity on T47D, MDA-MB-231, and SK-BR-3. Compounds 12 and 14 without cytotoxic activity on MCF-7 almost showed no activities on the other three cell lines. For the triple-negative breast cancer MDA-MB-231, Saponins 7 and 14 showed selective cytotoxic activity, 7 showed much more activity than 14 , suggesting the six saccharide units in sugar units and CH₃ on C-30 were the key moieties for the anti-proliferative activities. Further molecular mechanism of saponin 7 was studied on inhibiting cell proliferation of MDA-MB-231 cells. Saponin 7 could enhance apoptosis, arrest cell cycles, decrease mitochondrial membrane potentials (MMPs), and considered the involvement of reactive oxygen species (ROS) may explain this conundrum.