The invention relates to a replacement method for water-cooled ceilings of a mobile trolley of a 120-ton revolving furnace. The replacement method comprises the steps that the water-cooled ceilings are numbered online in advance, and each water-cooled ceiling has a unique position and a unique number; and according to the online arrangement and distribution condition, new water-cooled ceilings arearranged on a replacement part platform in the same online order in advance before online installation and then installed one by one according to the condition at the proper positions. The replacement method is simple and convenient to control, complex work of online ranking is completed offline, the phenomenon that different water-cooled ceilings are mixed in the installation process can be avoided, the replacing overhaul time is shortened, replacement work and production restoration can be completed within 72 hours, and the replacement method is high in efficiency, effective, beneficial topopularization and capable of reducing cost, increasing benefits and achieving circulation.