Development of novel magnetorheological elastomers based on magnetic nanoparticles is one of the important ways to realizing highly sensitive and flexible sensing. In this paper, a PDMS based flexible magnetic composite film based on Fe3O4 magnetic nanoparticles was prepared by using magnetic field induced growth method under magnetic field. The morphology and elements of the samples were characterized by digital microscopy and XRD. On a magnetometer and a self-built platform of magnetostriction measurement, the magnetic and magneto-mechanical properties of the samples were measured. The results show that the PDMS-based magnetic composite film in 10% concentration exhibits excellent in-plane magnetic anisotropic behavior under magnetic field induction, the saturation magnetization intensity can reach 5.9 emu/g at room temperature, which is 61% larger than that of the isotropic magnetic films prepared without magnetic field induction under the same condition, and the magnetic deformation angle can reach 80°. Finally, the nonlinear magneto-mechanical behavior of PDMS based magnetic composite films was proved theoretically by finite element analysis, which lays a theoretical foundation for testing the magnetic and mechanical properties of PDMS-based magnetic composite film.