Abstract: In order to study the mechanism of SO₂ generation and removal in circulating fluidized bed (CFB) boiler, a one-dimensional emission model of CFB was established based on the consideration of coal-S release, transformation and capture. It was verified on the 300MW subcritical CFB boiler. The effects of different parameters, such as primary air ratio, upper secondary air ratio, excess air coefficient, particle size distribution (PSD) and bed temperature on SO₂ emission were studied. The simulation results show that with the increase of primary air ratio and excess air coefficient, oxidation reaction of H₂S and COS in bottom of furnace increase and the total desulfurization efficiency improve. With the increase of the upper secondary air ratio, the SO₂ emission increases. As bed temperature increases, the desulfurization efficiency increases and the SO₂ concentration at furnace outlet decreases. When the desulfurizer PSD is thinner, the CaO reaction rate becomes higher, and the SO₂ concentration at the furnace outlet decreases.