It is significant to realize the efficient recovery of phosphorus resource and resource utilization of agricultural wastes. Three kinds of modified rice straw biochar (PRSB-Fe, PRSB-FS and PRSB-Ca) were prepared by modifying rice straw biochar (RSB) with FeCl₃, Fe₂(SO₄)₃ and CaCl₂, respectively. SEM, XPS, FTIR and BET characterized the modified biochar. Its adsorption capacity for phosphate in simulated wastewater and feces and wastewater separation solution of septic tank was explored by batch experiments. The results of simulated wastewater test showed that the adsorption of phosphorous by three modified biochar was well fitted to the pseudo-second-order kinetic model (R²>0.99), which indicating chemical adsorption was the major. The adsorption isotherms were more consistent with Freundlich equation at 20 ℃, indicating that multilayer adsorption was dominant. In addition, the coexistence of Cl^- had little effect on the adsorption capacity of modified biochar. For the feces and wastewater separation solution of septic tank with initial phosphorus concentration of (12.94±1.51) mg/g, pH value of 7.4±0.2, the adsorption capacity of PRSB-Fe-5, PRSB-FS-5 and PRSB-Ca-5 for phosphorus was 10.77, 23.35 and 0.85 mg/g, respectively. PRSB-FS-5 had the best adsorption of phosphorus, with the removal rate as high as 97.31%, and the residual phosphorus concentration was only 0.37 mg/L, reaching the first level A standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002).