Disclosed are an online analysis system and method for a line loss of a transmission line. The system includes: a terminal extension and a terminal host, where time information synchronization between the terminal extension and the terminal host and between terminal extensions is performed by a clock synchronization module, and communication between the terminal extension and the terminal host and between the terminal extensions is performed by a communications module; and a line loss management platform, configured to receive measurement data of the terminal extension and the terminal host, match time information in the measurement data, and if time information in the measurement data of the terminal extension and the terminal host is matched, and time information in measurement data of the terminal extensions is matched, determine corresponding line loss information based on corresponding measurement information.