The invention relates to a downhole slagging device for a small and medium cross-section vertical shaft, and belongs to the technical field of water conservancy and hydropower engineering. The device comprises a winch lifting system, an overhanging platform and slagging equipment, wherein the overhanging platform comprises two parallel rails and a sliding platform positioned at the upper side of the rails; each rail is divided into a fixed segment and an overhanging segment; the fixed segment of each rail is fixed to a rock step basement reserved at the top of the vertical shaft by virtue of a mortar anchor; the overhanging segment of each rail extends to be right above an opening of the vertical shaft, and the bottom of each overhanging segment is supported by a truss; the sliding platform is slidably connected with the two parallel rails; the winch lifting system comprises a winch and suspending points which are connected by virtue of steel wire ropes; the suspending points are arranged right above the overhanging segments of the two parallel rails. The device is applied to construction in the small and medium cross-section vertical shaft, is capable of improving the excavation efficiency, lowering the safety risk; shortening the construction period, lowering the construction cost and generating favorable economic and social benefits.