Valley injection plays a vital role in valleytronics application.A first principles study has been performed,demonstrating that a ferr emagnetic electrode can be used as a controllable valley-polarized-carriers injector,whose ON/OFF state can be easily switched only by changing the magnetic direction.Based on that,an ideal valley ON/OFF switch device is designed for future experimenters to generate,control and detect the valley-injecting current.Our scheme provides a gneral method which can be extended to other 2D crystals in furure applications.