Optical modulation of terahertz surface plasmon polaritons (THz SPPs) propagating in an intrinsic indium antimonide surface is demonstrated in this paper. The modulation is mediated by the modification of free carrier density with optical illumination. Simulation and experimental results show that a THz modulator can be realized by tuning the propagation lengths of THz SPPs, which could be controlled to be larger or shorter than the distance of two razor blades used for the coupling of the THz wave and the THz SPPs. In comparison with conventional THz modulation approaches, this method of manufacturing is simpler and the switching bandwidth is wider. The maximum modulation frequency of the modulators is anticipated to be above gigahertz, thus leading to the possibility of communication applications using the THz baseband.