In the era of carbon peak and carbon neutrality as the goal, 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, and the adsorption capacity for phosphate in simulated and actual wastewater was explored by batch experiments. The results of simulated wastewater showed that, the adsorption of phosphorous by three modified biochar was conformed to the pseudo-second-order kinetic model (R2>0.99), indicating chemical adsorption was the major. The adsorption isotherms were more consistent with Freundlich equation at 20 ℃, indicating that multilayer adsorption was dominant. Moreover, the coexistence of Cl- had little effect on the adsorption capacity of modified biochar. For the feces and wastewater separation solution of septic tank (initial phosphorus concentration of 12.94±1.51 mg/g, pH 7.4±0.2), the adsorption capacity of PRSB-Fe-5, PRSB-FS-5 and PRSB-Ca-5 for phosphorus was 10.77 mg/g, 23.35 mg/g and 0.85mg/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 for wastewater treatment plant. The study expected to provide reference for promoting the resource utilization of rice straw and achieving the “dual carbon” goals.