| In order to solve the that low coupling coefficient between coils and low charging efficiency for wireless charging system, a method for matching the radius of transmitting and receiving coil under different transmission distances was proposed. Firstly, equivalent circuit of the wireless charging system was established, and relationship between transmission efficiency and mutual inductance and internal resistance of the system was derived. Secondly, the planar loop coil was equivalently modeled, and the mutual inductance between two coaxial coils was obtained. Then, under the specific receiving coil, the relationship between average radius ratio λ and the transmission distance d was obtained by using the mutual inductance formula and the finite element simulation, and inside and outside radius of the transmitting coil was designed under the charging distance of the Automated Guided Vehicle. Finally, a comparison experiment of before and after optimization of the transmitting coil was carried out. The experimental results show that the optimized transmitting coil obviously improve the transmission efficiency of the system, and verify the correctness of the proposed design method.