The liquid crystal on silicon(LCOS) is one of the most appropriate spatial light modulators for holographic video display but the off-the-shelf LCOS is not suited because of mainly its small diffraction angle and low resolution. Metasurface, for example, gap-surface plasmon (GSP), has recently emerged as an innovative approach to control light propagation with unprecedented capabilities. In this paper, the feasibility of inserting one kind of metasurface into the LCOS was numerically discussed to deal with the problem. To the practical purpose, aluminum was used as the metal layer, the aluminum oxide layer was used as the insulating layer to form GSP structure. Firstly, the optical properties of aluminum at visible frequencies and the relative Fabry-Perot resonator model were studied. The preliminary structure then was inserted into LCOS to observe its effects on the electric field in the liquid crystal and consequently the changes of the liquid crystal director. The numerical simulations results show that the proposed structure has some influence on the diffraction of the far-field light and there are some changes in the viewing angle of the holographic display. The purpose of inserting the GSP into LCOS devices proposed here is technically feasible.