Abstract: A multi-relay wireless power transfer system includes many design parameters such as the coil self-inductance, the mutual inductance, the internal resistance, the operating frequency, and the compensation capacitance. Under the premise of meeting the load power requirements, it is still unclear how to optimize these parameters to design the system input and output characteristics and improve the system efficiency. Therefore, this paper proposes a coil-circuit parameter optimization design for the multi-relay wireless power transfer system with the goal of optimizing the transmission efficiency of the system and the constraints of simultaneously achieving constant voltage output and zero phase angle (ZPA) input characteristics. Firstly, based on a two-port network model, the realization conditions of the constant voltage output and the ZPA input, as well as the key factors affecting the transmission efficiency, are analyzed; Secondly, based on the product index of the coupling coefficient and the quality factor, the optimal number of the coils in a wide frequency range is analyzed; Based on the obtained optimal coil parameters, an optimal design for the system circuit parameters considering the constant voltage output and the ZPA input characteristics is proposed; Finally, based on the optimized coil and circuit parameters, a prototype of a five-coil wireless power transfer system is built. Experiments are conducted on the coil characteristics, the input output characteristics, and the system efficiency, verifying the effectiveness of the proposed parameter optimization design.