The invention provides an alarm method and system for preventing external damage to a power transmission line. The method comprises setting a detector early warning area of the power transmission line; arranging at least one pair of detectors according to the detector early warning area; and when an object enters the detector early warning area, sending an alarm signal and starting a camera to take pictures of the detector early warning area. The method and system can send an early warning signal when the object enters and can record a process that the object damages the power transmission line so as to facilitate the analysis of the process of line destruction and line tripping, and are accurate in the early warning result of preventing external damage to the power transmission line. A camera early warning area is also set. When the object enters the camera early warning area, the camera sends the alarm signal, and captures the process that the object enters the early warning area. Bysetting the camera early warning area, the method and system expand the early warning range of the power transmission line, and are accurate in the early warning result of preventing external damageto the power transmission line.