The invention discloses an underwater in-situ repairing device and a mounting method thereof,. The underwater in-situ repairing device comprises a floating air bag, a water inlet pipeline, a water purifying core and a water suction pump, wherein the floating air bag is of an annular structure, and the water inlet pipeline is vertically connected with the middle of the floating air bag in a penetrating way; the water suction pump is arranged at the upper end of the water inlet pipeline, and the water purifying core is arranged inside the water inlet pipeline; the water purifying core comprisesan upper cover plate and a lower tank body, and the upper cover plate is of a porous structure, and the lower tank body is peripherally closed; the upper end of the lower tank body is provided with anopening, and the lower end of the lower tank body is of a porous plate body structure; the upper cover plate is movably arranged at the upper end of the lower tank body, and the bottom of the lower tank body is positioned above the bottom of the floating air bag. The floating air bag, the water inlet pipeline, and the water purifying core are moavbly connected by an upper connecting cover and a lower limiting cover. The underwater in-situ repairing device disclosed by the invention has the advantages of long service time, simple and flexible structure, and high mounting and disassembly efficiency.