Aiming at the problems of slurry foaming and overflow in flue gas desulfurization system of high temperature circulating fluidized bed boiler in a company in Shanxi, the causes were analyzed and the improvement was made on the basis of the original process. The results show that the main causes of slurry foaming are the incomplete combustion of intermediate ash and the high concentration of organic matter in the water for desulfurization process. After improving the boiler operation condition and adding defoamer, the carbon content of fly ash in the boiler is reduced to 5%, and the circulating pump current returns to normal level. The desulfurization efficiency increased from 97.2% to 99%, the power consumption of circulating pump decreased from 380 kW·h to 316 kW·h, and the moisture content of desulfurization gypsum decreased from 16% to 9.5%.