The initial pH value of sludge affects sludge anaerobic digestion and dewatering performance. In order to determine the suitable pH value and decrease the sludge digestion time, in this study, the effect and the mechanism of combined acidification and anaerobic mesophilic digestion (acid-AMD) on sludge dewaterability were investigated. The changes and relationships among the extracellular polymeric substances (EPS), physicochemical properties, and rheological behavior of the treated sludge were analyzed. The results indicated that the combined acid-AMD treatment improved sludge dewaterability approximately 36.08% and 30.28% compared to the acid conditioning and AMD treatment, respectively. The factors improving sludge dewaterability include a lower sludge pH value and appropriate duration of AMD, changes in particle size, surface properties and distribution of the EPS fractions. The acid-AMD treatment hydrolyzed the EPS, loosening the sludge structure. These changes reflected in the rheological properties of the sludge. After the treatment, the network strength and colloid force of the sludge weakened. The linear viscoelastic region contracted, and the sludge system became sensitive to shear. These results demonstrated that rheological analysis can help explain the sludge dewatering mechanism. The acid-AMD treatment effectively changed the distribution of EPS that play a vital role in sludge dewaterability.