The invention belongs to the field of metallurgy technical equipment, and specifically relates to a method for online repairing a furnace-wall water cooling block of an arc furnace. The method for online repairing the furnace-wall water cooling block of the arc furnace comprises the following steps: 1) positioning and marking a water-leakage point of the water cooling block: online determining the position of the water-cooling-block leakage point, and marking on the pipe wall of a water cooling pipe, in the reverse direction of the water leakage point; 2) closing the water leakage points; 3) welding the water leakage points: cutting a welding hole at the marked place on the water-cooling pipe wall in the reverse direction of the water leakage point, welding the leakage point through the welding hole, and then welding the welding hole. The method for online repairing the furnace-wall water cooling block of the arc furnace helps to substantially reduce the risk caused by water-leakage-point welding by operating personnel entering into the furnace because of water leakage of the furnace-wall water cooling block, to make sure that the arc furnace welding keeps continuous stable and smooth running, to improve production efficiency and to reduce production cost.