Abstract: China has led the world in the field of ultra-high voltage transmission, but due to the impact of operating environment, ultra-high voltage transmission lines are prone to various safety or fault problems. Firstly, a brief overview is given of the online monitoring system for ultra-high voltage transmission lines, which is mainly divided into data acquisition layer, data transmission layer, and data application layer. Then, a detailed analysis will be conducted on the main online monitoring technologies used in the system, including climate and environmental monitoring technology, tower tilt monitoring technology, line icing monitoring technology, video image detection technology, etc. The application of these monitoring technologies can significantly improve the stability and reliability of the operation of ultra-high voltage transmission lines, providing users with more high-quality services.