To rapidly obtain a stable [email protected] heterogeneous Fenton catalyst, a novel in situ chemical co-precipitation method was developed. Compared with mechanical activation (MA)-pretreated cellulose (MAC), MA + FeCl3 (MAFC)-pretreated cellulose (MAFCC) was more easily dissolved and uniformly distributed in NaOH/urea solvent. MAFCC and MAC solutions were used as precipitators to prepare [email protected] and [email protected] nanocomposites, respectively. MAFCC showed stronger interaction and more uniform combination with Fe3O4 nanoparticles than MAC, implying that MAFC pretreatment enhanced the accessibility, reactivity, and dissolving capacity of cellulose thus, provided reactive sites for the in situ growth of Fe3O4 nanoparticles on the regenerated cellulose. Additionally, the catalytic performance of [email protected] nanocomposite was evaluated by using for catalytic degradation of methylene blue (MB), and [email protected] nanocomposite and Fe3O4 nanoparticles were used for comparative studies. [email protected] nanocomposite exhibited superior catalytic activity for the degradation and mineralization of MB in practical applications. After ten cycles, the structure of [email protected] nanocomposite was not significantly changed owing to the strong interaction between MAFCC and Fe3O4 nanoparticles. This study provides a green pathway to the fabrication of a stable nanocomposite catalyst with high catalytic performance and reusability for the degradation of organic pollutants.