Thermal hydrolysis characteristics of high solid content (7%, 9% and 13%) sludge were studied by thermal hydrolysis test. Anaerobic biodegradation of treated and untreated sludge and the relation of sludge flowability and sludge solid content were tested by biochemical methane potential (BMP) procedure and theological test, respectively. Results show that the dissolving ratio of solid organics increase with the prolonging of thermal hydrolysis time, and solid organics dissolve slowly after 30 min. Solid organics dissolving ratio of sludge with 7% solid content is higher than that of 9% and 13% sludge. The dissolving ratio of suspended solid (SS), volatile suspended solid (VSS) and COD of sludge with 7% solid content at 170 degrees C with 30 min were 43%, 47%, 42% respectively, and VFA/SCOD is 20.2%. Anaerobic biodegradation of treated sludge is improved. Methane production of 7%, 9% and 13% sludge at 170 degrees C with 30 min were 44%, 27% and 9% higher than that of untreated sludge. Methane production ratios increased 63%, 74% and 37% respectively. Methane content and methane production ratio of 9% sludge is higher than that of 7% and 13% sludge. Yield stress increases slowly with the increasing of solid content when solid content is lower than 9%, and it increases much faster and leads a steep decrease of sewage sludge flowability as solid content is higher than 9%.