Tunable terahertz plasmon in a graphene-based device with a grating serving as a top gate is studied.Transmission spectra exhibit a distinct peak in the terahertz region when the terahertz electric field is perpendicular to the grating fingers.Our results show that the extinction in the transmission of single-layer graphene shields beyond 80％.Electronic results further show that the graphene plasmon can be weakly adjusted by tuning the gate voltage.Theoretical calculation also implies that the plasmon frequency of graphene can fall into the terahertz region of 1-2 THz by improving the sustaining ability and capacitance of the top gate.