In this work, we investigate the quality factor and topologies of the series-parallel combined resonant circuit-based wireless power transfer system (SPRC-WPT system) and discuss the optimum design method of the SPRC-WPT system. The quality factor characteristics of the series-parallel combined resonant circuit (SPRC) are clarified and the way of obtaining a high quality factor in the SPRC is presented. A new concept of effective quality factor of SPRC is presented, and the four topologies of SPRC-WPT system are compared in the aspect of system quality factor, and the optimum topology is explored. The deduced theory is verified by electric circuit simulations with Pspice software package.