Abstract: In order to improve the adsorption capacity of cellulose to metal ion pollutants, this study prepared C6 carboxylic microcrystalline cellulose (CMCC) by selective oxidation system. The oxidation process and oxidation mechanism of CMCC were analyzed by using modern test and characterization techniques, and the adsorption capacity of CMCC for Cu²⁺ was studied. The results show that the HNO₃/H₃PO₄-NaNO₂ system selectively oxidized the C6 primary hydroxyl group on the pyranose ring in microcrystalline cellulose (MCC) macromolecule to carboxyl group by NMR and FTIR testing. The oxidation reaction etches the MCC surface to a certain extent, which improves the hygroscopicity of the CMCC, decreases the crystallinity, and reduces the thermal stability of the CMCC. Cu²⁺ adsorption experiments show that the adsorption behavior of CMCC follows the quasi-second-order kinetic model and Langmuir isotherm, and the saturated adsorption capacity of Cu²⁺ is as high as 165.5 mg/g. The adsorption thermodynamic analysis show that the adsorption of Cu²⁺ by CMCC is mainly through the chemical reaction between the carboxyl group and metal ions. The results indicate that the functional cellulose containing carboxyl active group can be used as a highly efficient adsorbent and will be widely used in the field of metal ion pollutant treatment.