In this study, the subwavelength grating multilayer structure with asymmetric boundary conditions is investigated using scattering matrix theory. A formal expression linking the reflection phase to the dielectric constant of the grating boundary medium is derived. Based on this expression, simulation experiments are conducted using the time-domain finite difference method to validate the pre designed silicon-based liquid crystal structure embedded with subwavelength gold gratings(G-LCoS). Through continuous optimization of the asymmetric boundary conditions of the subwavelength gold gratings, nearly 2π phase modulation across multiple wavelengths in the visible spectrum is realized via the G-LCoS. The formal expression and optimization process detailed in this paper offer reference for the design of devices with dynamic asymmetric boundary conditions, particularly complex amplitude modulation devices featuring nano/micro-scale structures. Additionally, this study contributes to a deeper understanding of these devices in terms of their physical mechanisms to a certain extent.