Multi-relay wireless power transfer (WPT) system has its unique advantages in power transfer over medium and long distance, which provides a new idea for energy supply of online monitoring device of high-voltage power transmission line (HVPL). However, the influence of embedded coils on the insulation performance of composite insulator is a problem we pay attention to. Based on the working principle and structure of multi-relay WPT system, the insulation performance of composite insulator with different sizes of weather shed is compared and studied. The surface potential drop, the distribution of surface and radial electric field intensity of insulator are obtained by finite element analysis (FEA) method. The simulation results show that the composite insulator with a certain thickness of weather shed featuring equal diameter represents better insulation performance.