We design an electronic control device based on graphene metamaterial with grating structure and study its polarization modulation properties in the terahertz region by finite element analysis method. The results show that in a wide band range of 0.1-2.0 THz, a broadband modulation occurs in the transmission of polarized terahertz wave parallel to the band (TE). It can be attributed to the intraband determined Drude conductivity of graphene. However, because of the plasma effect of graphene metamaterial, the transmission of polarized terahertz wave perpendicular to the band (TM) is almost transparent in the same terahertz region. More calculations also prove that with the increase of Fermi level of graphene, the modulation of TE will be enhanced largely while the plasma peak of TM will be blue shift. Meanwhile, the modulation can be further enhanced by the increase of the area ratio of graphene and the number of graphene layers in the device. In addition, the increase of the band width can cause a red shift of the plasma peak. With 2 μm period and 1.5 μm band width, a tunable maximum polarization degree of 0.89 can be achieved with 6 layer graphene device at Fermi level of 0.8 eV. This work paves a way for new graphene based terahertz polarization modulator.