A combined desulfuration, denitration, and demercuration apparatus and method using a semi-dry process in a circulating fluidized bed. The apparatus comprises a flue (2) and a circulating fluidized bed reaction column (8) connected in sequence, the flue (2) being connected to a bottom inlet of the circulating fluidized bed reaction column (8). The flue (2) is further connected to an ozone generation apparatus (1). The ozone distributor (1) feeds ozone in the flue (2) and sends oxidized flue gas into the circulating fluidized bed reaction column (8), so that the SO2, high-valence NOx, and Hg2+ contained in the flue gas are removed through reactions with a Ca-based absorbent under the effect of atomized water in the reaction column (8).