As a technology which can enhance charging safety and flexibility, wireless power transfer (WPT) was applied in some high-tech equipment, especially in aquatic drones, considering its complex charging environment. In order to improve the stability of wireless power transfer system voltage output while the relative position of two coils changing, a WPT circuit which can supply stable 12 V output voltage and current various with the distance changing between transmitting coil (Tx) and receiving coil (Rx) in a range from 5 mm to 30 mm is designed. To get the maximum transmission efficiency, the system is supposed to keep working at resonant frequency which is realized by frequency control chips for inverter in first side and rectifier in second side respectively. The detailed study of battery charging current versus coil distance is analyzed by simulation and experiment. In addition, the parameters of different circuit components such like capacity and inductance are fixed to obtain the resonance, which is determined by calculation and simulation. Meanwhile, the shape of coils is considered as well because it has clear affection on mutual inductance.