When the transmission line passes through the mountains, rivers and other complex regional environments, it usually uses 2G/3G communication, resulting in poor quality of data transmission because the signal is weak even no signal. However, in these areas, the wireless frequency band of 5.8G is small, which has little effect by rain attenuation and suitable for long-distance transmission with the coordination of directional antenna. Therefore, the 5.8G wireless network communication is the best choice. In this paper, an on-line monitoring system for transmission line based on 5.8G is proposed, and the power supply mode and safety protection are analyzed in depth. This design has been successfully used in pilot applications and solves the problem of transmission line operation data access in non-line of cable resources or no network coverage area, to ensure the effective and reliable operation of the system.