The work aims to generate ferric oxide (Fe3O4) on the surface of cellulose nanocrystals (CNC) by co-precipitation method, and construct magnetic hybrid particles Fe3O4@CNC, and combine them with polyvinyl alcohol (PVA) to prepare magnetic composite films. Fe3O4 was in-situ generated on the surface of CNC by co-precipitation method, and then the hybrid particles were combined with PVA to prepare magnetic composite films by solution compounding method. The morphology, mechanical properties, thermal stability, ultraviolet protection and magnetic properties of the hybrid particles and the composite films were studied. The dispersion of hybrid particles in the matrix was improved by in-situ loading, the tensile strength and modulus of the composite films were improved, and the films showed excellent UV protection and magnetic properties. Fe3O4@CNC/PVA magnetic composite films with excellent mechanical properties and UV protection are successfully prepared in this study, which have certain application potential in electromagnetic protective packaging and other fields. The research results provide a new idea and method for the development and application of magnetic composite films, and have broad application prospects.