Extraordinary optical transmission is the widely known phenomenon of enhanced transmission of waves through subwavelength periodic metallic apertures. Here, we propose efficient terahertz transmission modulation in subwavelength metallic slits by a graphene plasmonic ribbon array. The extraordinary optical transmission through the subwavelength metallic slits can be tuned by coupling with the plasmonic resonance of graphene ribbon array, resulting in a deep transmittance depression. Numerical simulations show that maximal transmission modulation depth of 98.99% can be obtained at 1.03 THz via this mechanism.