In order to find the optimum limestone addition rate for the desulfurization during fluidized bed sludge incineration, a simulation is conducted using a thermodynamic equilibrium model. For the analyses of the desulfurization and the pollutant emissions by the limestone addition, the Ca/S molar ratio is changed tip to 7.0 with the consideration of the CaO bound in the ash of the sludge. According to the simulation, it is found that the desulfurizing reaction with limestone is greatly influenced by phosphorous content in the ash. If the Ca/P molar ratio is less than two, the desulfurizing reaction between CaO and S may not occur. The reason is that the reactivity of CaO with a P component in the ash is superior to that with an S component.