Two kinds of optical modulation processes are designed based on nested split-ring resonators in the terahertz regime. By photo-conductivity induced mode switching effect, the resonant structure and resonant mode will be changed, and the two contrary tunable resonant properties are obtained. Without illumination, the two designs both have three resonant peaks. As the intensity of illumination increases, for the first design, the first and the third resonant peaks disappear and a broadband resonant peak at the second resonant frequency is formed eventually; however, the second design realizes an exactly opposite modulation process. Moreover, in order to identify the modulation mechanism, we discuss and analyze the distributions of the electric fields and surface currents in detail. Our designs can be implemented in tunable terahertz functional devices and provide important reference value for the design of terahertz metamaterial.