The development of terahertz (THz) technology is creating a demand for devices that can modulate THz beams. Here, we propose a novel THz modulator based on patterned graphene/gallium nitride Schottky diodes hybridized with metasurfaces. Ultrasensitive dynamic multidimensional THz modulation is achieved by changing the Schottky barrier of the heterojunction, shifting the Fermi level between the Dirac point, changing the conduction band and the valence of graphene via continuous-wave optical illumination or bias voltages. When the Fermi level is close to the Dirac point, the modulation is ultrasensitive to the external stimuli. Applying an optical power of 4.9–162.4 mW/cm² or a bias voltage of 0.5–7.0 V, the modulation depth initially increases, then decreases, and the phase difference linearly increases, therein the maximum modulation depth is 90%, and the maximum phase difference is 189°. In short, the proposed THz modulator has potential application in ultra-sensitive optical devices.