Semidry flue gas desulfurization ash (FGDA) is a kind of solid waste, which is mainly composed of CaSO3 and produced by coal-fired power plants. Thermal decomposition has been considered as a promising method for FGDA treatment. However, strict equipment requirements and the high energy consumption brought by the high decomposition temperature do hinder the industrial application of the thermal treatment methods. In order to solve these issues, the mechanism of FGDA thermal decomposition was investigated, with emphasis on the effect of additives, impurities, and reaction conditions. The results showed that the temperature, reductant of carbon powder, and the impurity of Ca(OH)(2) in the FGDA promoted the decomposition efficiency of CaSO3, while CaCO3 in the FGDA had a negative effect on CaSO3 decomposition. An optimal increase of the thermal decomposition efficiency of FGDA was achieved by using carbon powder as a reducing additive. Furthermore, the decomposition temperature of FGDA was decreased from 1100 to 1000 degrees C. The decomposition efficiency of CaSO3 increased from 55.56% to 89.48% at 1000 degrees C. The content of active CaO in the roasting product reached 40%, which is a potential desulfurizer.