A terahertz(THz) wave optical modulation system was introduced. The modulation was based on the coupling of THz waves and THz surface plasmon by a simple slit between a razor blade and a semiconductor wafer. The modulation process was realized by changing the illumination intensity on the intrinsic semiconductor surface. With or without the optical illumination, the plasma frequency of the semiconductor was larger or smaller than the frequencies of the surface plasmon. Therefore, the THz surface plasmon propagating on the semiconductor and the THz wave coupled from the surface plasmon can be switched on and off. In comparison with conventional THz modulation approaches, this method has the advantages of wide modulation bandwidth, high speed, low cost and room temperature operation etc., which are favorable to THz wave communication applications. The simulation and experimental results prove the feasibility of the THz wave modulation system.