To elucidate the compositional characteristics and targeted anti-cancer molecular mechanisms of Rosa sterilis pomace polyphenols, this study utilized UPLC-MS/MS technology for the precise identification of their phenolic profiles. By comparing the components with those of normal Rosa roxburghii pomace polyphenols, the material basis for the antioxidant and tumor-suppressive effects of Rosa sterilis was objectively revealed. Based on this, their antioxidant activities were analyzed using Peroxyl Radical Scavenging Capacity (PSC) and Oxygen Radical Absorbance Capacity (ORAC) assays. Furthermore, using triple-negative breast cancer MDA-MB-231 cells as a model, their anti-proliferative effects and molecular mechanisms were multidimensionally investigated by combining wound healing, Transwell invasion, flow cytometry, and RT-qPCR assays. The results demonstrated that the total phenolic content of Rosa sterilis pomace (H-TP) was significantly higher than that of normal Rosa roxburghii, and this core difference primarily stemmed from the elevation in its free phenolics (H-FP) content. H-FP contained more abundant catechins and phenolic glycoside derivatives, exhibiting the most potent antioxidant capacity. Cellular experiments confirmed that H-FP significantly inhibited the proliferation of MDA-MB-231 cells (with an EC50 value of 2.21 mg·mL-1), induced S-phase cell cycle arrest, and promoted early cell apoptosis. Additionally, H-FP significantly suppressed the tumor cell healing rate and drastically decreased the number of invading cells from 170.2 to 10.2. Molecular pathway studies revealed that H-FP exerted potent anti-cancer effects by upregulating the expression of key genes such as CHK2 and Bax, while downregulating AKT, PI3K, and CDK2, relying on the multi-target synergistic effect of its polyphenolic components. In summary, this study clarifies the multi-pathway inhibitory capabilities of Rosa sterilis pomace polyphenols against triple-negative breast cancer cells, providing a theoretical foundation for the high-value utilization of pomace resources and the development of natural anti-tumor functional foods.