Planar core is added to the magnetic resonant wireless power transmission system to increase the self-inductance and the coupling coefficient between the primary coil and secondary coil, and to enhance the transfer power and efficiency. Based on the equivalent circuit analysis, mathematical expression of system parameters are given by the circuit differential equations. Furthermore, a prototype of wireless power transmission system is built, and experiments including coupling coefficient, operating frequency, misalignment, and power transfer characteristic of the system are conducted. The system realizes 1.85 kW power transmission with 10 cm air gap and 80％ efficiency. By using the planar core, the coupling mechanism can achieve stable and effective transmission in 25％ misalignment, and its volume is smaller.