In research on triple-negative breast cancer (TNBC), autophagy is recognized as a mechanism that promotes tumor cell survival under chemotherapy, and thus, inhibiting autophagy has become a potential therapeutic strategy to overcome drug resistance. We discovered Platycodin D2 (PD2), a natural saponin compound derived from the root of Platycodon grandiflorum, can inhibit autophagic flux in various breast cancer cells (MCF-7, MDA-MB-231, 4T-1), characterized by the accumulation of LC3II/I and P62 proteins. To further explore the pharmacological mechanism of PD2, we observed the inhibition of autophagosome-lysosome fusion by PD2 using MCherry-GFP-LC3 plasmids, leading to massive accumulation of autophagosomes. By observing lysosomal pH levels, we speculated that this fusion blockade was due to the decrease in lysosomal acidity. To investigate PD2's potential as an anti-tumor drug, we examined its cytotoxicity on breast cancer cells and found that it inhibits cell growth and synergizes with the clinical drug Lapatinib. We also found that PD2 inhibits DNA synthesis and induces apoptosis through EdU staining experiments and detection of Cleaved PARP/Caspase 9/Caspase 3. In animal experiments using Balb/c mice, PD2 inhibited 4T-1 TNBC cell growth and enhanced the therapeutic effect of Lapatinib. Overall, our study elucidates the pharmacological effects of PD2 in vitro and in vivo, suggesting it is a potential natural drug for treating breast cancer. The underlying mechanisms warrant further investigation.