Abstract: The excessive Cl^- of absorption tower slurryin limestone-gypsum desulfurization system is a frequent problem in thermal power plants. High chlorine content will cause great harm to equipment,which causes low desulfurization efficiency and sulfur dioxide emissions exceeded. The Cl^- of absorption tower slurry in a thermal plant suddenly rising,which exceeds the operation specified value of wet flue gas desulfurization（ WFGD） system 20 000 mg/L. In order to find out the cause of sudden rise,the Cl^- source of absorption tower slurry are analyzed. The experimental results show that the main reason for the sudden rise is the high content of Cl^- in economic coal. After adopting measures such as coal blending adjustment,slurry replacement and other methods,Cl^- of absorption tower slurry tends to equilibrium condition. Desulfurization system returns to regular and stable operation,which provides references for solving the problem of excessive concentration of Cl^- of absorption tower slurry in limestone-gypsum desulfurization.