In order to improve the physical layer security of visible light communication（VLC） system, a physical security method based on liquid crystal reconfigurable intelligent surface（LC-RIS） assistance is proposed. LC-RIS with controllable refractive index is introduced into the receiving end of the traditional VLC system, and the difference between the legitimate channel and the eavesdropping channel is increased by adjusting the refractive index of the incident optical link with applied electric field, so as to increase the reachable safe transmission rate of the VLC system. The simulation results show that compared with the traditional VLC system, the reachable safe transmission rate of the proposed VLC system is increased by 1 bit/ channel use, and the security performance of the communication is effectively improved. LC-RIS index optimization algorithm based on particle swarm optimization （PSO） algorithm has good robustness.