Under strong alkaline condition, magnetic O-site carboxymethyl chitosan was prepared through a series of reaction such as carboxymethylating reaction, anhydriding reaction and alcoholysis reaction at 60 ℃, using monochloroacetic acid,chitosan, acetic anhydride and CoFe₂2O₄ nanoparticles as reactants, using pyridines and anhydrous sodium acetate as catalyst, and characterized via FT-IR, SEM. energy dispersive spectrum(EDS), XPS and vibration sample magnetometry. The series of results show that magnetic O-site carboxymethyl chitosan was prepared successfully, and the as-prepared magnetic O-site carboxymethyl chitosan is cluster, and its saturation magnetization is 27.6 Am²/kg.