Distributed line fault diagnosis system, as the key product application of EHV transmission line fault location service, is of great significance to the safety, reliability and economic operation of power system. This paper combines the development and application of practical system, and presents a novel diagnostic service system based on distributed installation of transmission line measuring terminals, online monitoring and maintenance system and mobile terminal application service. The key implementation schemes of products and systems are described in this paper. In addition, some key technologies such as energy harvest and management, high-speed sampling and synchronization, time synchronization and coordinate positioning, traveling wave start-up and identification processing based on transmission line measuring terminal, dual servers hot backup redundancy and data security isolation, ranging algorithm and analysis, GIS information and micro-meteorological service technology based on online monitoring and maintenance system, and application security service of mobile terminal are proposed. The system products described in this paper have successfully passed the test and certification organized by the China Electric Power Research Institute, and run steadily and reliably on the EHV AC1000 kV transmission line in the western of China. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 524609170021).