Spatial light modulator (SLM) is the core device of holographic display, which requires a large space-bandwidth product (SBP), especially needing a wide viewing angle. According to the grating theory, the scale of the holographic display unit should be close to the wavelength of light. The transmission resonances of deep metallic sub-wavelength grating structure, which is produced by the surface plasmon and Fabry-Perot (FP) resonance based on metal grating phenomenon of Wood's anomaly, especially the metal-insulator-metal (MIM) structure provides a theoretical and effective technique for enhancing the reflection resonances and can be used for implementing the holographic display unit technology. In this paper, we replace the top electrode layer of the LCOS with a metallic deep sub-wavelength grating structure and change the grating period, slit width and spacer thickness. The simulation results by aid of CST software are given, which demonstrate that the improved device with dielectric medium parameter within liquid crystal refractive rate range (1.4~1.7) can reach 0 to 2π phase modulation in the visible wavelength range. Moreover, it also decrease the difficulty of device processing.