Most of terahertz (THz) systems and their application require THz modulator to modulate THz signals. Photoelectric modulators at present are made and producted by semiconductor-based, such as GaAs, materials. But unfortunately, due to their limited response to THz wave, these modulators based on traditional semiconductors are not suitable for THz systems. To overcome the limitation on modulation bandwidth and depth, a new scheme of THz wave intensity modulation system was proposed, which was based on all optically tuned graphene on silicon(GOS). This new modulation system was realized by photo-induced free carrier, which can absorb THz waves. By controlling light intensity which incident on modulator, photo-induced carrier density could be tuned, and even transmitted THz wave intensity was modulated. In this work, the fundamental properties of this THz intensity modulator was theoretically and experimentally studied , such as modulation depth and bandwidth. Our experiment results also show that it has a wide modulation bandwidth, which can modulate efficiently THz wave in THz-TDS of frequency range from 0.1 to 2.5 THz, and the typical modulation depth is 12 % at an optical pump fluence of 18 mW/mm2. what's more, the modulation depth increases with increasing optical pump fluence.