The invention provides a middle joint replacing device used for replacing a middle joint in a blast furnace air supply system. The device comprises a forklift base, a supporting plate, a left clampingbase, a right clamping base, a left protection arm and a right protection arm. The supporting plate is perpendicular to the forklift base, one side of the supporting plate is fixed to the forklift base, the left clamping base is arranged on the side, away from the supporting plate, of the forklift base, the right clamping base is parallel to the left clamping base and arranged on the side, away from the supporting plate, of the forklift base, one end of the left protection arm is fixedly connected with the end, away from the forklift base, of the supporting plate, one end of the right protection arm is fixedly connected with the end, away from the forklift base, of the supporting plate, a preset angle is formed between the left clamping base and the forklift base, the left clamping base is in clamped connection with a first flange, and the distance between the left protection arm and the right protection arm is not smaller than the outer diameter of the middle joint. By means of the technical scheme, the middle joint replacing device is used in cooperation with a forklift, 3-4 h of overhaul time can be saved, the labor intensity of workers is greatly lowered, and the problem thatsafety accidents happen in the up-down lifting process when a chain block is used is avoided.