This paper presents an optimal design of the moveable ball-shaped transmitting coil used for wireless charging. Firstly, the mathematical model of mutual inductance calculation is established. Based on this model, the rates of changes of 9 kinds of coils windings’ mutual inductance are compared and the optimal design of the coils is selected by using the Maxwell (2D) simulation. Secondly, the topology of the wireless power transfer system is designed and simulated in the Matlab. Finally, an experimental platform is built and the results are compared with those of the theoretical calculation and simulation to verify that the proposed wireless power transfer system is feasible.