Ball milling is a prevalent technique for synthesizing phosphorus/carbon (P/C) anodes, conventionally deemed inherently safe due to the presumed avoidance of toxic white phosphorus (WP) formation. Challenging this widespread assumption, this study firstly reveals the overlooked generation of WP during the P/C ball milling process, highlighting a significant and unrecognized hazard. To counteract this critical issue, an effective sulfur (S₈) co-ball milling strategy is introduced, which not only suppresses WP formation but also promotes phosphorus polysulfide (P₄S_n) generation. Critically, P₄S_n converts in situ to Na₃PS₄ solid electrolyte upon initial sodiation, establishing an integrated three-dimensional (3D) ionic network throughout the electrode matrix. This facilitates rapid Na⁺ transport, especially beneficial for high-mass-loading electrodes. Consequently, the S-P/C electrode with 3.0 mg·cm⁻² demonstrates superior rate capability (424 mAh·g⁻¹ at 5.0 A·g⁻¹) and excellent cyclic stability (86.9% capacity retention after 1000 cycles at 5.0 A·g⁻¹) in half-cells. Furthermore, sodium-ion full cells pairing this anode with a high-loading Na₃V₂(PO₄)₃ cathode (~ 12.0 mg·cm⁻²) demonstrate remarkable electrochemical performance.