Abstract: Online monitoring technology can realize real-time monitoring of overhead transmission lines,channel environment,meteorological conditions,wind deviation and galloping,which effectively ensure the safe and stable operation of overhead transmission lines. However,some towers are in the area covered by no operator base station,so online monitoring cannot be realized. An online monitoring data transmission method in no-signal area based on unauthorized spectrum terminal direct communication technology（D2D）was proposed.Firstly,the distribution characteristics of online monitoring devices for overhead transmission lines were analyzed,the spectrum resources suitable for transmission line environment were selected,and a path loss model of direct communication between devices was established.Then,the channel access mechanism based on listening before sending was adopted to realize the friendly coexistence with existing communication technologies. Finally,based on the transmission line channel environment in the no-signal area,the transmission route based on directional antenna was used for information relay. The simulation results show that the proposed method can reduce the transmission power of the antenna,improve the survival time and stable working time of the whole monitoring network,and ensure the high-speed and reliable return of online monitoring data.