To improve the safety and reliability of transmission lines, the principle of intelligent early warning system based on transmission line on-line inspection system is researched. Transmission line on-line inspection system replaces manual inspection by using visual supervisory, infrared imaging, OPGW optical-electric separation technique and EPON communications technology. On this basis, an intelligent early warning system using hierarchical structure is proposed. This system has two stages. The first stage is located in the central processing unit on each tower, which is used to preliminarily judge a fault state or an abnormal state of transmission lines. The second stage is located in backend host of the on-line inspection system, which is used to accurately determine the type of transmission line fault. This intelligent early warning system can realize the fault state or abnormal state of transmission lines effectively.