Abstract: The research on the influence of the ash from circulating fluidized bed (CFB) boiler on the desulfurization behavior of coal pyrolysis in the cogeneration process which combines coal pyrolysis with CFB combustion was carried out in a fixed bed reactor. The circulating ash with CaO from CFBC shows a significant action of sulfur retention in coal pyrolysis process. Compared with the coal pyrolysis with inert solid heat carrier, about 60%~70% sulfur in gas and tar is retained in the ash. It is revealed that the behavior of sulfur retention increases with the atomic ratio of Ca/S. Only 0.6% and 4.8% of the total sulfur enter in the gas phase and liquid phase respectively at Ca/S atomic ratio of 3.6. At the same time, CaO can reduce the yield of tar and the total sulfur content in the tar. It is predicted that CaO can play a positive role in the reduction of sulfur emission in the combination process of coal pyrolysis and CFB combustion.