It is very important to develop high performance electro-optic modulator to construct on-chip photonic circuits. A novel surface isoplasmon waveguide electro-optic modulator based on graphene mixing is designed, which consists of bias-silicon nanowires, double graphene layers, and silver nanowires placed between the two graphene layers. A two-dimensional finite-difference time-domain algorithm is used to calculate and analyze the influence of structural parameters on the modulation performance of the device. The simulation results show that the designed modulator can achieve excellent modulation performance at the operating wavelength of 1550 nm, and its 3 dB modulation bandwidth is as high as 250 GHz. The modulation depth and power consumption are higher than 0.15 dB/μm and lower than 11.5 fJ/bit, respectively. This modulator can provide design ideas for the development of a new generation of high-performance integrated electro-optic modulators.