As a beneficial supplement to radio frequency communication, Visible Light Communication (VLC) can provide high-speed, ultra-low delay, green, low-cost, safe and reliable communication services. However, limited coverage range has become the bottleneck of long distance and reliable communication in VLC system. Since the coverage range of VLC is limited, an effective method is proposed to use relay technology in VLC system. The paper starts from the functional characteristics of relay technology, and analyzes the relay system model. Then we give a review on several key technologies of relay-assisted VLC, including the application scenario, network mode, transfer agreement, modulation mode, working mode and non-orthogonal multiple access. The reliable performance, safety and energy efficiency with the corresponding key technologies are analyzed. Finally, we point out a research emphasis and direction in VLC by adopting the heterogeneous network, non-orthogonal multiple access, multi-input and multi-output, full-duplex mode multiple and relays collaboration for the next generation communication network.