Abstract: The reaction of carbothermal decomposition of calcium sulfate( CaSO_4) generally occurs in circulating fluidized bed boilers that use furnace desulfurization. It is of great significance to in-depth study the reaction mechanism for improving the sulfur fixation performance in the furnace. Based on the experiment of thermogravimetric and mass spectrometry,the equivalent characteristic spectrum( ECSA) method was used to deeply explore the reaction process of graphite thermal decomposition of CaSO_4. Besides,combined with the horizontal tube furnace experiment,the solid product after the reaction was characterized. The temperature range of the main reactions in the decomposition of CaSO_4 was comprehensively analyzed. The results show that based on the experimental data of thermogravimetric mass spectrometry combined with ECSA,the main reaction process of graphite thermal decomposition of CaSO_4 can be well analyzed. The selfdecomposition reaction of CaSO_4 occurs at about 1 100 ℃,and the addition of graphite reduces the decomposition of CaSO_4 temperature to about 900 ℃,while increasing the fastest decomposition rate and shortening the complete weight loss time. At about 1 050 ℃,the rate of thermal decomposition of CaSO_4 by graphite is the fastest,but the temperature corresponding to the maximum decomposition rate is not correlated with the amount of graphite added. With the increase of the molar ratio of graphite and CaSO_4 increases,the fastest reaction rate of the decomposition of CaSO_4 gradually increases,and the temperature of thermal decomposition of CaSO_4 decreases slightly and tends to be stable. In the process of thermal decomposition,the reaction of CO_2,SO_2 and Ca O is the most important reaction product. When the reaction temperature is higher than 1100℃,the reaction between graphite and residual CaSO_4 will also occur to generate Ca S and CO_2,but to a small extent.