The invention discloses a prestress lengthening device and a prestress lengthening method, and relates to the technical field of bridge construction, the lengthening device is arranged in an area withthe minimum
hogging moment of a
continuous beam bridge, the two sides of the lengthening device are a first area and a second area respectively, and the lengthening device comprises a plurality of first prestress ribs and second prestress ribs; bridge decks which are connected in sequence, at least one first prestressed rib is anchored on one side, far away from the first area, of each
bridge deck, and at least one second prestressed rib is anchored on one side, far away from the second area, of each
bridge deck; meanwhile, the first prestressed tendons are arranged on the first area and thebridge
deck slab in a penetrating mode, the ends, on the
bridge deck slab, of the first prestressed tendons are first tensioning ends, and the other ends of the first prestressed tendons are first anchoring ends; the second prestressed tendons are arranged on the bridge
deck slab and the second area in a penetrating mode, one ends of the second prestressed tendons on the bridge
deck slab are second anchoring ends, and the other ends of the second prestressed tendons are second tensioning ends. Anchoring points and tensioning points can be arranged in a segmented and scattered mode, the purposeof lengthening prestress is achieved, and meanwhile the requirement for the thickness of the bridge deck slab is lowered.