Two dimensions (2D) materials play more and more important roles these days, due to its broad applications in many areas. Here, we proposed an optically pumped terahertz (THz) modulator, based on Si-grown MoS2 nanosheets. The broadband modulation effect has been proved by THz time domain spectroscopy and numerical simulation. The modulation depth of this Si-grown MoS2 nanosheet can reach over 75% under the low pumping power of 0.24 W/cm2, much deeper than that of bare silicon. By theoretical model and simulation, it is proved that the broadband modulation effect can be described as a free carrier absorption for THz waves in the Drude form. Importantly, a catalyst mechanism in the Si-grown MoS2 is concluded that MoS2-Si heterostructure make the MoS2 can catalyze more carriers generated on the Si surface. This novel 2D material has a high effective modulation on THz waves under a low pumping power density, which gives itself a bright potential in THz applications.