Abstract: Agricultural biomass waste can be expected to efficiently treat for environmental protection. Conventional agricultural waste disposal can often include direct incineration, direct landfilling, and simple composting. Among them, the direct incineration has caused air pollution; landfilling has also led to the waste of land resources and environmental contamination; while the simple composting can struggle to effectively handle the various microorganisms and pathogens, leading to pests and diseases after application. Agricultural waste, such as straw and sawdust, can be converted into the biomass activated carbon with an excellent pore structure rich in surface functional groups after heat treatment. The adsorption performance can be improved for agricultural production after the modification and preparation of multifunctional materials. Particularly, capacitive deionization is one of the main application directions of the biomass-activated carbon. Some challenges have remained in the structure of the biochar. However, the existing biochar cannot fully meet the demand for the high-performance electrodes of the capacitive deionization. It is generally required for sufficient surface functional groups and a better match between physical and chemical properties, rather than the single pore distribution in the electrodes. This study aims to systematically explore the effects of the one-step activation, two-step activation, element loading, and oxidation on the physicochemical properties, surface functional group composition, and electrochemical properties of the biochar materials. The peanut straw was taken as the raw material. The removal effect of the humic acid and Cu²⁺ was further explored in the capacitive deionization. The most suitable biochar preparation was achieved after optimization. The results showed that the two-step activation was more conducive to the synergistic effect of the oxidative modification and phosphorus doping than the one-step activation, indicating the longer reaction depth and the richer P-O functional groups. Compared with the impregnation oxidation, the HNO₃ hydrothermal and air oxidation shared the milder modification strength. The oxidation reaction was completed without damage to the carbon skeleton. The N, P, and O elements were introduced to improve the pore structure of the biochar for the high adsorption; Compared with the unmodified biochar, the specific surface area of the N-P-O co-doped biochar increased by up to 10.61 times; The N-P-O co-doped biochar (APCNW) exhibited an increase in the mesopore volume from 0.013 to 0.078 cm³/g after the two-step activation with the HNO₃ hydrothermal oxidation, compared with the phosphorus-loaded biochar (APC); The one-step activation N-P-O co-doped biochar (PCNW) showed an increase in the mesopore volume from 0.033 to 0.042 cm³/g, compared with the phosphorus-loaded biochar (PC) after the one-step activation; Better mesoporous structures of the materials were obtained for the ion diffusion paths; Specific capacitance magnitude was one of the most important indicators to measure the capacitive deionization of the materials; After the introduction of N, P, and O elements under the current density at 1 A/g, the specific capacitance of the APC increased from 93 to 190 F/g for the APCNW, with an increment reaching 104.3%; The HA deionization capacities of the APCNW and PCNW were 10.92 and 10.06 mg/g, respectively; There was the increase by 110% and 147%, respectively, compared with the 5.2 mg/g for APC and 4.07 mg/g for PC; The Cu²⁺ deionization capacities of the APCNW and PCNW were 53.83 and 36.02 mg/g, respectively; There was some increase by 99% and 223%, respectively, compared with the 27.01 mg/g for APC and 11.15 mg/g for PC; The capacitive deionization of the biochar electrodes were attributed to the complexation of the phosphorus-containing functional groups with the humic acid, and the coordination between O and P atoms on the biochar surface and Cu²⁺. Among them, the APCNW shared an intact carbon skeleton structure and abundant surface P-O groups; The combined proportion of the -COOH/P-O-P and C=O/P=O functional groups reached 53.1% in the O 1s deconvolution peaks; The optimal capacitive deionization was achieved among all the materials.