Due to the inconsistency between the maximum allowable conductor temperature for long-term operation and the conductor temperature taken into sag calculation for the tower design, and the influence of creep caused by the increase of operation life as well as the growth of surrounding forest and vegetation, the problem of conductor temperature rise and sag off-limit of the heavy-load transmission line in summer high temperature weather cannot be ignored. For this reason, the safety indexes consist of temperature rise margin and clearance margin that represent the safe operation of transmission lines are proposed according to the electrothermal coupling process of “current increase→conductor temperature rise→sag increase→clearance distance reduce”. The estimation method of safety indexes based on weather monitoring and numerical weather forecasting is established. The operating temperature and sag off-limit warning method of the heavy-load transmission section under the N-1 operation mode is proposed. The feasibility and effectiveness of the proposed method is verified through the accident inversion of a tree-touch fault caused by the temperature and sag off-limit of a transmission line. The results can help power grid operators to check the safety margin of the overhead transmission lines in high-temperature weather of summer and prevent discharge fault caused by tree contact due to the insufficient of insolation clearance and cascading fault inducted by power flow transfer of transmission section. This work is supported by Science and Technology Project of State Grid Corporation of China (No. SGSDDK 00KJJS1800210) and National Natural Science Foundation of China (No. 51707018).