Abstract: The response surface methodology was applied to optimize the preparation of magnesium modified peanut shell carbon (Mg-PBC), which enabled the co-adsorption of ammonia and phosphorus. A systematic investigation was conducted on the co-adsorption characteristics and mechanism of Mg-PBC. The findings indicate that the pyrolysis temperature (ranging from 400 to 600 ℃), pyrolysis duration (1-3 h), and Mg²⁺ concentration (1-3 mol·L^-1) not only altered the physicochemical properties of the biochar but also exerted a remarkable impact on its adsorption efficiency. The adsorption of NH₄⁺-N and PO₄^3--P conformed to pseudo-second-order kinetic model (q_e=31.97 and 98.80 mg·g^-1). According to the adsorption characteristics, adsorption kinetics and adsorption isotherm results, the main adsorption mechanisms were determined to be struvite precipitation, surface functional group action, electrostatic attraction and ion exchange. The removal amount of nitrogen and phosphorus by the four treatments followed the order of Mg-PBC > Mg²⁺≈BC+Mg²⁺»BC. When the molar ratio of nitrogen to phosphorus was set at 1.2, and the Mg-PBC dosage was 3 g·L^-1, the removal efficiencies of phosphate and ammonia in the wastewater from soybean product processing reached to 72.82% and 67.04%, respectively.