To improve the capability of radiation control with new devices，an active modulator at terahertz frequencies is designed with metamaterials. The structure under investigation is three dimensional. By adjusting the bias voltage applied on the substrate，the carrier recombination as well as the effective electromagnetic parameters are effectively altered，and the modulation of the structure is realized. The research shows that the modulator can control and operate THz radiation effectively and in real-time. The transmission modulation reaches 75% with the frequency shift more than 0.1 THz，which is better than that of structures with two dimensional designs，providing a novel way to construct new types of THz devices.