Abstract: The efficient treatment of agricultural biomass waste has always been the core issue in the field of environmental protection, traditional agricultural waste disposal methods include direct incineration, direct landfilling, simple composting, etc. However, direct incineration causes air pollution; landfilling leads to waste of land resources and environmental contamination; while simple composting struggles to effectively handle various microorganisms and pathogens, and may trigger issues such as pests and diseases after application.The conversion of agricultural wastes such as straw and sawdust into biomass activated carbon with excellent pore structure and rich surface functional groups through heat treatment technology can not only improve its adsorption performance, but also create additional benefits for agricultural production through modification and preparation of multifunctional materials. As one of the main application directions of biomass activated carbon, capacitive deionization technology is still facing the challenge that the structure of biochar is difficult to meet the demand for high-performance electrodes—the existing biochar generally has problems such as single pore distribution, insufficient surface functional groups, and mismatch between physical and chemical properties and the demand for capacitive deionization electrodes. Based on this, in this study, the effects of one-step activation process, two-step activation process, different element loading methods and different oxidation methods on the physicochemical properties, surface functional group composition and electrochemical properties of biochar materials were systematically compared with peanut straw as raw material, and the removal effect of humic acid and Cu²⁺ in capacitive deionization technology was further explored, aiming to optimize the most suitable biochar preparation process. The results showed that the two-step activation process was more conducive to the synergistic effect of oxidative modification and phosphorus doping than the one-step activation, and its reaction depth was deeper and the P-O functional groups were richer. Compared with impregnation oxidation, HNO₃ hydrothermal oxidation and air oxidation have a milder modification strength, and the oxidation process can be completed without damaging the carbon skeleton.The introduction of N, P, and O elements can improve the pore structure of biochar and enhance its adsorption effect; compared to unmodified biochar, the specific surface area of N-P-O co-doped biochar can be increased by up to 10.61 times; through two-step activation combined with HNO₃ hydrothermal oxidation treatment, the obtained N-P-O co-doped biochar (APCNW) exhibited an increase in mesopore volume from 0.013 cm³/g to 0.078 cm³/g compared to the phosphorus-loaded biochar (APC) prepared by the two-step activation method; through one-step activation treatment, the one-step activation N-P-O co-doped biochar (PCNW) showed an increase in mesopore volume from 0.033 cm³/g to 0.042 cm³/g compared to the phosphorus-loaded biochar (PC) prepared by the one-step activation method; better mesoporous structures make the materials have better ion diffusion paths; specific capacitance magnitude is an important indicator measuring the capacitive deionization capability of materials; under the condition of current density at 1 A/g, the introduction of N, P, and O elements made the specific capacitance of APC increase from 93 F/g to 190 F/g for APCNW, with an increment reaching 104.3%; the HA deionization capacities of APCNW and PCNW were 10.92 mg/g and 10.06 mg/g, respectively; compared to 5.2 mg/g for APC and 4.07 mg/g for PC, they increased by 110% and 147%, respectively; the Cu²⁺ deionization capacities of APCNW and PCNW were 53.83 mg/g and 36.02 mg/g, respectively; compared to 27.01 mg/g for APC and 11.15 mg/g for PC, they increased by 99% and 223%, respectively; this indicates that the complexation of phosphorus-containing functional groups with humic acid, and the coordination between O, P atoms on the biochar surface and Cu²⁺, further improve the capacitive deionization capability of biochar electrodes.Among these, APCNW has an intact carbon skeleton structure and abundant surface P-O groups; in the O 1s deconvolution peaks, the combined proportion of -COOH/P-O-P and C=O/P=O functional groups reaches 53.1%; it exhibits the optimal capacitive deionization performance among all other materials.