To mitigate riverine pollution caused by phosphogypsum accumulation during phosphate mining, we developed an iron-based/calcium-oxide (CaO) composite capable of removing phosphorus from wastewater within 40 s. Static batch tests first quantified the phosphorus-removal capacity of CaO alone, followed by a comparison of three iron systems—microscale zero-valent iron (mZVI), mZVI + Fe²⁺, and mZVI + H₂O₂ + HCl. Subsequent combined-dosing experiments confirmed the composite's suitability for rapid phosphorus removal under simulated flood-season conditions, achieving outstanding efficiency within 40 s. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) revealed a dense surface layer of iron–calcium phosphate precipitates. Synergy and cycling tests demonstrated excellent reproducibility and long-term stability. Overall, this work provides a practical solution for the rapid remediation of high-phosphorus effluents during flood events and promotes sustainable phosphorus recovery.