Transmission lines are long exposed to nature environment and are vulnerable to adverse weather conditions such as lightning, typhoon, rainstorm and so on. Different measures should be taken according to different fault causes. This paper studies the fault probability of transmission line under different weather conditions, and proposes to assess the N-m risk under forecasting weather condition for those high fault risk lines based on the analysis results, then chooses the operation mode to reduce the system risk and enhance the ability of defense the adverse nature disasters. Through statistical analysis of the distribution characteristics of adverse weather in a southern China area along the coast, getting the frequency and degree of adverse weather in different time, and through testing on RBTS, the feasibility of the proposed method is verified.