To solve the power supply problem of the high-voltage side detection module in HVDC applications, a multi-relay wireless power transfer (WPT) system with an operating frequency of 800 kHz is designed. This paper establishes the model of multi-relay WPT system and derives the expressions of the efficiency and output power. By analyzing the influence of the insulator metal flanges and optimizing the coil spacing, the energy transfer efficiency of the system can be improved. A 1.1m prototype with 8 resonant coils and 2 metal flanges is built and the experimental results verify the influence of metal flanges on the multi-relay WPT system and the feasibility of optimized arrangement of coils.