The invention relates to a live-line replacement method and device for the jumper suspension insulator string of a common-tower and multi-circuit power transmission line. By means of an insulated pull rope, an insulated plug board is suspended at a working point. By means of an insulated operation rod, the insulated plug board is clamped between two insulators. Through pulling the insulated pull rope, an insulator string is locked inside a U-shaped groove in the insulated plug board by a movable clamp board (6). After that, the original insulator string is removed. The insulated plug board is in the form of a U-shaped board (4). The two arms of the U-shaped board (4) are respectively provided with one rotatable movable clamp board (6). One end of a spring (9) is connected with the rotatable movable clamp board (6), and the other end of the spring (9) is connected with the U-shaped board (4). The U-shaped board (4) is further provided with a fixed pulley (8). Each movable clamp board (6) is provided with a pull hook part. By means of the device, suspension strings on the jumper of a common-tower and multi-circuit power transmission line can be safely and effectively replaced. Therefore, the potential safety problem that might occur due to the adoption of a middle-phase cross arm and a lower-phase cross arm by an operator can be avoided.