Controlling the propagation of terahertz (THz) wave is very important in the application of THz technology. THz modulator is considered to be the key device in next-generation THz wireless communication. A new-type optically pumped THz modulator based on Si-grown monolayer tungsten disulfide (WS₂) was demonstrated. WS₂ acts as a catalyst due to the formation of heterojunction at the junction between Si substrate and WS₂, since more carriers are catalyzed at the heterojunction under pumping power, the modulator can achieve more deep modulation depth. The result shows that the modulation depth of this Si-grown monolayer WS₂ modulator can reach 63.6% under the low pumping power of 117 mW and the wavelength of the pump light is 660 nm. This novel two-dimensional transition metal dichalcogenides with high modulation efficiency have a great prospect in the application of THz technology.