Graphene is a special two-dimensional material, and has excellent physical properties. Combining graphene with micro-nano devices has become a hot research topic. In the mid-infrared to terahertz band, surface plasma can be excited in graphene, which can be applied to multi-functional tunable devices. In this paper, we propose a broadband light modulator working in the infrared waveband, which combines graphene plasma with silicon based subwavelength metallic gratings. By applying a bias voltage on graphene to change the Fermi level, the transmission light can be modulated within the wavelength range from 7 to 22 μm, and the modulation depth can reach up to 99.96% (33.77 dB).