Dynamically controlling terahertz (THz) waves with an ultracompact device is highly desired, but previously realized tunable devices are bulky in size and/or exhibit limited light-tuning functionalities. Here, we experimentally demonstrate dynamic modulation on THz waves with a dielectric metasurface in mode-selective or mode-unselective manners through pumping the system at different optical wavelengths. Quasi-normal-mode theory reveals that the physics is governed by the spatial overlap between wave functions of resonant modes and regions inside resonators perturbed by pump laser excitation at different wavelengths. We further design/fabricate a dielectric metasurface and experimentally demonstrate that it can dynamically control the polarization state of incident THz waves, dictated by the strength and wavelength of the pumping light. We finally numerically demonstrate pump wavelength-controlled optical information encryption based on a carefully designed dielectric metasurface. Our studies reveal that pump light wavelength can be a new external knob to dynamically control THz waves, which may inspire many tunable metadevices with diversified functionalities.