Based on the metamaterial structure, a terahertz electro-optic modulator operating at the frequency of 0.34 THz is designed, manufactured and characterized in this paper. The modulator loads voltages through the metamaterial structure and obtains a strong electric field at the opening of the metamaterial structure to transport the photon-generated carrier generated by the silicon under constant laser irradiation. By modulating terahertz waves with the voltage modulation signals, the modulation depth is increased from 9% to 50%, and the modulation speed is up to 30 kHz.