Resonator design methods play an important role in realizing efficient wireless power transfer via magnetic resonant coupling (WPT-MRC) systems. In WPT-MRC systems, transmitting (Tx) resonator arrays are often used to extend the range of the power supply. However, it is difficult to design a receiving (Rx) resonator that prevents significant drops in transfer efficiency across the Tx resonator array owing to the complexity that stems from fluctuating coupling coefficients. In order to resolve this difficulty, this paper proposes a genetic algorithm-based Rx resonator array design method. Using the proposed design method, the computed minimum transfer efficiency of a power supply system using a Tx resonator array increased from 3.8% to 42.6%, and the computed results were verified by simulations and measurements.