The invention discloses a semi-dry process flue gas desulfurized ash modification treatment method. Flue gas desulfurized ash, which is obtained after a CFB desulfurization technology, is modified by MnO2, water, and an oxidant; during the modification process, flue gas desulfurized ash is continuously oxidized, calcium oxide in flue gas desulfurized ash is converted into calcium hydroxide, calcium sulfite is converted into calcium sulfate, and modified desulfurized ash is obtained. Specifically, the modification treatment comprises the following steps: spraying atomized water, which contains a MnO2 catalyst, on desulfurized ash, stirring for 5 to 10 minutes to uniformly mix desulfurized ash and water; then spraying an oxidant on the desulfurized ash, stirring for 25 to 40 minutes, and evenly mixing so as to fully modify desulfurized ash. According to the modification treatment method, atomized water containing MnO2 and an oxidant are combined to modify desulfurized ash in an enclosed environment; after modification, the water content of desulfurized ash is 15 to 17%, desulfurized ash can be directly entrucked without dehydration or drying, and the energy and time are saved.