The utility model discloses a device for forcedly oxidizing and converting byproduct ammonium sulfate produced by flue gas desulfurization, which comprises an ammonium sulfate crystallizing section, a SO2 absorbing section and a mist and ammonium eliminating section, wherein the ammonium sulfate crystallizing section, the SO2 absorbing section and the mist and ammonium eliminating section are successively arranged from the bottom up, the ammonium sulfate crystallizing section is provided with a flue gas inlet, an ammonia water adding device and a oxidant adding device are arranged in a slurry tank under the flue gas inlet, a primary cyclic spray device is arranged above the flue gas inlet, and the primary cyclic spray device is connected with the slurry tank via a circulating pump. The utility model is rationally added with a strong oxidant (ozone) and ammonia water, which strengthens the oxidation of (NH4)2SO3, increases the concentration of ammonium sulfate in the slurry, and avoids the secondary pollution produced by concentration and drying of the ammonium sulfate. The utility model reasonably utilizes the heat energy of the flue gas, and reduces the quantity of steam used for drying the ammonium sulfate.