Hydrothermal treatment coupled with mechanical expression at increased temperature in two separate cells respectively is effective for the dewatering of excess sludge with low energy consumption. The objectives of this study were to evaluate the dewatering performance and the characteristics of obtained products (hydrothermal sludge, hydrochar and filtrate). The results showed that harsher hydrothermal treatment (temperature from 120 to 210 °C and residence time from 10 to 90 min) led to greater water removal (from 7.44 to 96.64% reduction of total water) and mechanical pressure became less significant as it increased. The whole expression stage was completely described by the modified Terzaghi-Voigt rheological model. The role of tertiary consolidation stage in the water removal was reduced with hydrothermal treatment being stronger. The hydrothermal treatment is mainly a devolatilization process. The observed changes in H/C and O/C for hydrothermal sludge suggested dehydration was the major reaction mechanism and decarboxylation only occurred significantly at higher temperature. The higher heating value correlated well with carbon content of sludge, which was increased by 4.8% for hydrothermal sludge at 210 °C for 60 min and significantly decreased by 15.4% for hydrochar after 6.0 MPa for 20 min. The solubilization and decomposition of proteins, polysaccharides and DNA were determined to be temperature and residence time dependent. The improvement of dewaterability was closely correlated to the variation of these biopolymers. The filtrates collected above 150 °C were found to be acidic. The increase of humic substances and the melanoidins formed by Maillard reaction were largely responsible for the filtrate color.