The magnetically coupled system is important to the magnetically coupled resonant WPT system. The power transmission capacity and overall efficiency of WPT system will change greatly with the change of its quality factor. Therefore, the optimized design of the magnetically coupled system plays an important role in changing the overall efficiency of the WPT system. This paper takes the WPT system with an S/SP compensation network as the research object. The single-turn simulation model of the magnetically coupled system is analyzed, and then the multi-turn coils electrical parameters model of the magnetically coupled system is deduced. On this basis, the relationship between the loss of the magnetically coupled system and the coil turns is established, and an optimized design method for the coil turns with constant output power is proposed. Finally, the feasibility of the proposed theoretical method is verified by the WPT system prototype experiments.