Magnetic coupling resonant WPT technology has demonstrated significant potential for powering online monitoring devices on high-voltage transmission lines due to its capability for long-distance transmission and high power delivery. This paper proposes a novel multi-coil coupled WPT system based on a two-stage domino series structure, which decomposes a complex 5 m long-distance multistage magnetic coupling system into two shorter 2.5 m segments, substantially reducing the computational complexity of cross-coupling analysis. The multi-coil power transfer resonator adopts a planar structure embedded within epoxy resin boards, forming a layered insulation configuration that not only mitigates the influence of high-voltage electric fields on the WPT system but also functions similarly to a grading ring to improve electric field distribution. Experimental results demonstrate that the proposed system achieves an output power of 10.2 W with a transmission efficiency of 6.3% at a 5 m transfer distance.