The invention discloses a power transmission line online monitoring system adopting distributed power supply. The system includes a field monitoring terminal, a communication network and a control center. The field monitoring terminal is intended for real-time acquiring operation state of a power transmission line, transmitting the acquired operation state of the power transmission line to the control center through the communication network. The control center is intended for computing, analyzing and storing the received operation state of the power transmission line. The field monitoring terminal includes a field monitoring mainframe, a monitoring unit and a power supply unit. The field monitoring mainframe includes a data acquisition circuit, a micro-processor, a storage unit, a first wireless communication module, a second wireless communication module, a communication switch module and a power supply interface circuit. According to the invention, by monitoring the operation state of the power transmission line through the field monitoring terminal and transmitting the acquired operation state of the power transmission line to the control center through the communication network, the system realizes remote monitoring of the power transmission line, lowers labor intensity, and ensures safe operation of the power transmission line.