Herein, a sub-wavelength gold (Au) grating is sputtered onto an indium tin oxide electrode of a classical liquid crystal on silicon (LCoS). After it is mounted on a thin liquid-crystal cell with a bottom aluminum electrode, a composite resonant waveguide structure, namely GLCoS, is created. In contrast to LCoS that operates based on liquid-crystal propagation effects, in the GLCoS, the surface plasmon of the upper electrode is resonantly coupled with a TM-Fabry-Pérot (TM-FP) resonator in the grating slits to induce a phase modulation of zero-order reflection. The aluminum electrode acts as a reflective backing plate and contributes the waveguide with the Au grating and thin liquid-crystal cell, which enhances the resonance coupling. While acting as an optical device to control the wavefront, the GLCoS also operates as an electronic control device, applying voltage to change the refractive index of the liquid crystal cell; it can control the dielectric condition of the open cavity FP at the boundary to achieve an active 0--2π phase modulation. The experimental results show that the proposed GLCoS structure can be used in phase-spatial light modulators with the 1-μm-level pixel, and it has prospects for application in holographic video displays with high spatial bandwidth product.