The study evaluated the influence of thermal hydrolysis pretreatment (THP) on anaerobic digestion (AD) ability of high solid sludge. The transformation characteristics of organics during the THP + AD process of dewatering sludge from wastewater treatment plant was investigated using a lab-scale THP reactor and four anaerobic digesters. The reduction efficiency of volatile suspended solids using THP + AD exceeded 49%. The acceleration of biogas production during AD was due to the enhancement of protein hydrolysis and acidogenesis by THP. THP had only minimal influence on the improvement of carbohydrate acidogenesis. The hydrolysis of poly phosphates was likely the main reaction of phosphorus transformation. Biochemical generation of sulfide and ammonia nitrogen occurred during the acidogenesis.