The invention provides a power transmission line multi-energy complementary power supply system and a power supply method thereof. The system comprises a storage battery power supply branch, a CT induction power supply branch, a solar power supply branch and a lightning protection ground wire coupling power supply branch, the storage battery power supply branch, the CT induction power supply branch, the solar power supply branch and the lightning protection ground wire coupling power supply branch are arranged in parallel, each power supply branch is connected with a BMS management system anda power bus, the lightning protection ground wire coupling power supply branch, the solar power supply branch and the CT induction power supply branch are each provided with a power supply state detection unit, the storage battery power supply branch is provided with an electric quantity detection unit and a charging management unit, a voltage detection unit, the power supply state detection unit,the electric quantity detection unit and the charging management unit are all connected with the BMS management system, the BMS management system is connected with a monitoring platform, and the monitoring platform is connected with a monitoring mobile terminal. The power supply of the four power supply branches is coordinately controlled through the BMS management system, and the power utilization safety of power transmission line monitoring equipment is ensured.