Theoretically, the magnetic coupling resonance wireless power transmission system can be designed to achieve high efficiency within the critical coupling zone. However, the system efficiency will decrease sharply when the distance is in the weak coupling zone and the resonance frequency will split when the distance is in the strong coupling zone. Therefore, a planar spiral coupling coil with ferrite core is designed and fabricated. Then the frequency tuning system is designed to keep the high transmission efficiency even when the transmission distance changes in the strong coupling zone. Measurement results demonstrate, when the transmission distance changes between 5-20 cm, the designed wireless power transmission system can guarantee an efficiency above 80%.