The invention relates to a fixture for replacing a strain insulator string under an electrified state based on an earth potential method. The fixture comprises left and right linking plate clamps which are respectively used for connecting linking plates at two ends; the left and right linking plate clamps are respectively composed of a mounting base, a pulley block and a set of linking plate clamping connectors; the pulley blocks are mounted on the mounting bases; the linking plate clamping connectors are detachably connected with the mounting bases; the left and right linking plate clamps are connected through a high-strength rope which is wound between the two pulley blocks; one end of the high-strength rope is fixed on one of the linking plate clamps while the other end is served as a free operating end. According to the fixture and an operation method provided by the invention, the operation is simple, quick and convenient; the working strength of operators can be reduced; the working efficiency is increased; the operation time is reduced; the applicability is excellent; the operation can be efficiently and safely finished under the condition of ensuring the safety of operators; the safe running of a power grid is ensured while the power-off/on operation volume and the change of the running mode of the whole power grid are reduced; the running safety and reliability of the power grid are greatly increased; the economic benefit is obvious.