This paper proposes a downhole contactless power coupling system characterized by high efficiency, low noise, compact size, vibration-resistance and easy-to-maintenance. The system includes a power transformer with detachable structure. High-frequency soft switch techniques are used for power inverting circuit. In addition, resonant point of inverting circuit can be locked automatically through regulating of switch frequency in accordance with closeness to zero of phase difference between output voltage and current of inverting circuit. In this way, deviations in resonant points induced by environmental temperatures can be eliminated. Test results show that electrical energy transmission efficiency of the prototype can be maintained well above 70%. With high reliability, the system can be used to replace conductive slip ring to achieve electrical energy transmission from rotary drill pipe to guide sleeve. Application of the system may facilitate miniaturization of drilling tools. Compared with conventional drilling tools, the power coupling rotary steering tool may effectively reduce drilling cost, enhance ROP, reduce sticking and promote borehole quality. Consequently, it is significant for horizontal drilling.