High levels of heat shock protein 70 (HSP70) in tumors are commonly associated with poor prognosis, enhanced doxorubicin (DOX)-induced cardiotoxicity, and even drug resistance in DOX-related cancer chemotherapy. Several peptides possess remarkable protein inhibition and chemosensitization effects, which are attributed to their specific targeting ability against HSP70. However, the inherent poor cell penetration capacity considerably restricts the biomedical applications of these peptides. We herein describe the design and development of anti-MUC1 aptamer-peptide conjugate (ApPC) as targeted chemosensitizers to overcome the above-mentioned issues. Moreover, DOX could be loaded on the ApPC to deliver the DOX-enclosed agent ApPC-DOX, which simultaneously acts as a targeted chemosensitizer and anticancer agent for combating drug resistance in breast cancer therapy. This innovative, engineered biocompatible conjugate not only enhances the sensitivity of DOX-resistant cells but also alleviates cardiotoxicity of DOX in vivo, highlighting the success of this targeted chemosensitizer strategy.