The invention provides +/-800 kV extra-high voltage line duplex double-hanging-point suspension string insulator replacing method, The method includes assembling a bypass stress system; installing a bypass stress system; anchoring an auxiliary hardware string tool on a cross arm side extension plate; tightening the bypass stress system; after the bypass stress system is adjusted, detaching the insulator string to be replaced, and then falling the insulator string to the ground and replacing the whole string; after the insulator string is replaced, adjusting the large-load mechanical tightenerto loosen the bypass stress system, and detaching the bypass stress system and the auxiliary hardware string tool. The insulator string can be replaced in the shortest time, and the replacement is convenient, rapid and safe.