There are a lot of problems such as bad environment adaptability, low degree of intelligence, multidisciplinary cross and more difficulty for power line inspection with large unmanned helicopter, at present unmanned helicopter patrol relies mainly on artificial ground operation, in order to improve inspection efficiency and safety, it is of great significance to study automatic inspection technology with large unmanned helicopter. This paper puts forward the concept of full automatic inspection, gives analysis on necessity of automatic inspection, discusses the key technologies on safety inspection of over low altitude and beyond visual range, including real-time differential positioning, communication relay, autonomous obstacle avoidance, high precision measurement of position and attitude, automatic photoelectric tracking, flight plan and path planning, task planning and control. Automatic inspection technology is used in transmission line inspection of power grid in China for the first time, inspection results show that this technology is advanced and effective. Automatic inspection technology represents an efficient, intelligent, and new power inspection mode, the development and progress of the technology has a significant role in promoting intelligent operation and maintenance level of power grid in China.