Despite the great progress in the control of primary tumor growth, metastasis remains the major challenge of breast cancer therapy in clinics, which is highly related to the upregulation of reactive oxygen species (ROS) and overexpression of its relevant pro-survival miR-155 gene. Therefore, we fabricated a poly-antioxidant (FTP) to deliver anti-miR-155 for synergistic treatment of metastatic breast cancer by ROS scavenging and miR-155 inhibition. FTP was synthesized by the polymerization of fluorated-polyethyleneimine (FPEI) and antioxidants (TEMPOL), using a glutathione (GSH) responsive linker for controlled drug release. Notably, the poly-drug strategy could not only promote the tumoral accumulation of small molecular antioxidants but also enhance the transfection efficiency of anti-miR-155 owing to the hydrophobic property of TEMPOL. After synergistic treatment, the NF-κB pathway was significantly blocked, thereby generating strong anti-metastatic ability both in vitro and in vivo. The poly-antioxidant could be a new type of nanoplatform for highly efficient and safe miRNA delivery, which also provides a promising strategy for the synergistic treatment of metastatic breast cancer.