For the surface-plasma holographic display scheme of refractive index modulation, the relationship between the structural parameters of the variable-refractive-index thin-film sinusoidal grating and the diffraction efficiency is simulated by using the rigorous coupled-wave calculation method. The results show that when the silver is used as the surface plasmon excitation material, 0.2 μm is the optimum thickness of the grating. On this basis, the diffraction efficiency of the first-order light is proportional to the refractive index difference and the period. The refractive index range is expanded, confirming that when the refractive index modulation range is 1.34~1.8 and the period is 0.9 μm, the diffraction efficiency can reach more than 33%. This study has reference significance for the design of surface plasmon holographic display structures.