Ag nanoplates and Fe3O4 nanoparticle-based ferrofluids were utilized to fabricate a magnetic field controlled optic switch. The changing of light transmittance (LT) is caused by the rotation of Ag nanoplates, whose long axis always follows the orientation of external magnetic field to minimize the potential energy. The sensitivity of switching was optimized by choosing Ag nanoplates with appropriate size and concentration. The switching of transmission is proved to be fast and fully reversible. This phenomenon not only indicates an effective method to adjust the propagation of optical signals, but also reveals the possibility and great potential to develop magnetic controlled functional devices.