A novel phosphorus-based alloy composite, Sb–CuP2–C, is synthesized by a simple two-step high-energy ball milling (HEBM) method and used as an anode material for sodium-ion batteries. The HEBM process with Sb, Cu, P, and C generates Sb and CuP2 phases instead of forming Cu2Sb and P owing to the differences in binding energy among the elements, which is confirmed by density functional theory. The as-prepared Sb–CuP2–C composite consists of the electrochemically active components Sb and CuP2, which are embedded in a conductive carbon matrix, as determined by X-ray diffraction analysis and high-resolution transmission electron microscopy. The composite electrode demonstrates significantly improved cycling performance owing to the presence of both a carbon matrix, which acts as a buffer to accommodate the large volume expansion during sodiation, and a conductive metal framework of copper. The introduction of red phosphorus into the composite yields higher reversible charge capacities compared to that of a Cu2Sb–C composite. The Sb–CuP2–C electrode exhibits a high capacity retention of ∼80% even after 100 cycles. Moreover, it presents stable rate cycling performance with a capacity retention of ∼78% at 3000 mA g−1.