A metal nanowire array electro-optic modulator with high modulation coefficient is designed based on surface plasmon resonance (SPR) technology. The reflectivity of reflection spectrum is changed by changing the driving voltage, and electro-optic modulation is achieved. The finite difference time domain (FDTD) method is used to optimize the parameters of metal nanowire array and buffer medium layer, and the influence of structural parameters on reflectivity of reflection spectrum is investigated. Results show that when the metal nanowire diameter is 34 nm, buffer dielectric layer thickness is 1600 nm, the mode coupling ability of electro-optic modulator is the strongest. With the gradually increasing of driving voltage, the total absorption peak of reflectance spectrum appears obviously shift to the right. Electro-optical modulation of the optical signal by electrical signal can be achieved by adjusting the driving voltage loaded on the electrode.