Abstract: Metamaterial refers to some of the art ificial structures, and it represents superior physical properties that natural material do not have and it can control electromagnetic waves in novel way. This paper analysed modulation mechanism of a single split double ring structure metamaterial in theory, and by applying differential equation and method of equivalent circuit to solve the general mathematical expression of resonant frequency of the modulator with geometric parameters of itself. Different modulation frequency of geometric parameters of modulator with Matlab was calculated. The modulator was simulated theoretically using CST (microwave studio), the numerical simulation of the resonance absorption intensity at 0.775 THz, 0.95 THz and 1.65 THz could reach 70%, 65% and 68% respectively. The structure of singly split double ring metamaterial can be used as a THz modulator.