Abstract: The effects of desulfurization ash and ferric chloride as conditioners on the sludge dewatering performance were investi-gated with the capillary suction time and the water content of the sludge cake as evaluating indicators. Extracellular polymeric substance measurements and Fourier transform infrared spectroscopy were used to analyze the sludge dewatering mechanism. The results show that a significantly better treatment effect can be obtained with desulfurization ash and ferric chloride together than just one conditioner. In the treatment process, a large amount of the tightly bound extracellular polymeric substance is spalled by desulfuriza-tion ash and ferric chloride, in which a part turns into a loosely bound extracellular polymeric substance and a slime extracellular polymeric substance, and the other part is removed by ferric hydroxide, which effectively decreases the water content of the sludge cake and the capillary suction time. Pearson correlation analysis results indicate that the tightly bound extracellular polymeric substance has significant correlation with the capillary suction time and the water content of the sludge cake, so it is an important factor affecting the sludge dewatering performance. Infrared spectrum analysis of the sludge filtrate reveals that hydrolysis happens when the extracellular polymeric substance enters into the sludge supernatant, and the contents of amino acid, aliphatic acid and other small molecule organic matter increase. When the dosages of desulfurization ash and ferric chloride are 300 mg·g^-1 and 60 mg·g^-1, respec-tively, the capillary sop time and the water content of the sludge cake decrease to the lowest. Their minimum values are 14.3 s and 70.22%, falling by 98.48% and 16.10%, respectively.