In order to explore the joint remediation technology of eutrophic water bodies and jointly solve the problems of sediment pollution, plant waste and excessive phosphorus in water bodies, preparation of modified biochar by co-pyrolysis of sediment and plant waste to adsorb phosphorus in water was studied. The effects of preparation conditions such as the ratio of biochar raw material, pyrolysis time and temperature, modifier and its ratio on the adsorption of phosphorus were studied. The effects of pH and co-existing ions on the adsorption of phosphorus by Fe modified biochar were investigated. The results show that the adsorption capacity of Fe modified biochar 20:10:3-ZnBC450-3Fe on low concentration phosphorus (1 mg/L) was 2.76 mg/g, and the removal rate was 68.96%. Under neutral conditions, Fe modified biochar adsorbed phosphorus well. Na⁺, K⁺ and Cl⁻ promoted the adsorption of phosphorus, but Al³⁺, ${\mathrm{C}\mathrm{O}}_3^{2-}$ and ${\mathrm{S}\mathrm{O}}_4^{2-}$ inhibited the adsorption of phosphorus. The adsorption of phosphorus by Fe modified biochar conformed to the quasi-second-order kinetics and Freundlich adsorption isotherm model. SEM-EDS test proves the effectiveness of phosphorus adsorption by iron-modified biochar. The preparation of biochar by co-pyrolysis of sediment and plant waste to adsorb phosphorus in water is a comprehensive restoration technology combining physical, chemical and biological means for river and lake garden ecosystem, which has good application prospect.