Nitrogen and phosphorus co-doped activated carbon-supported iron catalyst (Fe/AC-NP) was synthesized from dicyandiamide, NaH₂PO₄ and FeCl₃•6H₂O via two-step impregnation-evaporation method, and subsequently employed for the catalytic degradation of piperazine in aqueous solution. The microstructure, chemical composition and textural properties of Fe/AC-NP were characterized by XRD, SEM, XPS and BET. The effects of raw materials ratio and heteroatom doping type on the chemical oxygen demand (COD) and ammonia nitrogen contents of the catalytic piperazine oxidation degradation by Fe/AC-NP were investigated, while the possible reaction mechanism was analyzed through radical quenching experiments, water contact angle measurements and XPS analyses. The results showed that the Fe/AC-NP prepared with n(C₂H₄N₄)∶n(NaH₂PO₄)=2∶1 displayed the best properties, with a high specific surface area of 699.647 m²/g, a crystal lattice spacing of 0.0169 nm, and a uniform distribution of iron species without the formation of large agglomerates. The Fe species were successfully loaded and coordinated with N and P atoms, predominantly existing in the forms of Fe(Ⅱ) and Fe(Ⅲ). The optimal reaction conditions of Fe/AC-NP for piperazine degradation were temperature 230 ℃, pressure 1.2 MPa, catalyst dosage (mass concentration, the same below) 1.25 g/L, under which, COD was completely removed within 70 min, and the ammonia nitrogen removal efficiency reached > 88.7% after 150 min. Even after five successive recycles, the COD and ammonia nitrogen removal efficiencies remained high at 96.3% and 84.2%, respectively. The synergistic contribution of graphitic and pyrrolic nitrogen functionalities enhanced oxygen adsorption. The catalytic degradation process was primarily driven by hydroxyl radical chain reactions and facilitated by electron transfer between the carbon support and metal sites. Furthermore, phosphorus doping modulated the surface acidity/basicity and stabilized Fe—N bonds, enabling the N-P co-doped carbon matrix to maintain catalytic activity via electronic regulation throughout the reaction cycles.