Abstract: Mechanically dewatered sewage sludge generally has a moisture content that ranges from 80% to 85%. This high level of moisture makes it necessary for this type of sludge to be dewatered and dried to facilitate the disposal. Bio-drying of sewage sludge based on thermophilic aerobic fermentation is an economical and energy-saving method for sludge disposal. During the bio-drying process, microbial water, which plays an important role in the final efficiency of sewage sludge bio-drying, is generated by the degradation of organic matter. Accordingly, the investigations of dynamic variations in organic composition and water generation are essential to the management of sewage sludge bio-drying. Therefore, the aim of this study was to investigate the degradation of organic matter and the generation of water during the sewage sludge bio-drying process. To accomplish this, a bio-drying experiment was conducted and the data were analyzed using the water mass balance equation. The bio-drying process was conducted using an auto-control technology for 20 d, during which the pile was aerated intermittently using an air blower. In addition, the pile was turned on the 9th, 12th, 15th and 18th day. The overall process consisted of 4 phases which in turn were the temperature increasing phase, the first thermophilic phase (>50 ℃), the second thermophilic phase, and the cooling phase (<50 ℃). On-line measurements were used to determine the water vapor and aeration water input. Additionally, the levels of hydrolyzable matter, lipid, lignocellulose and humic acid in different stages were also determined. The results showed that the total water generation was 61.80 kg/t for bio-drying material based on the water mass balance equation. The order of water generation rates calculated was as follows: the first thermophilic phase > the temperature increasing phase > the second thermophilic phase > the cooling phase. The dynamic variations in water generation were as follows: during the first thermophilic phase, water generation peaked at 9.40 kg/(t·d) on the 3rd day, while its mean value in the first thermophilic phase was 6.51 kg/(t·d). After the first even turning on the 9th day, the water generation increased to its second peak of 8.40 kg/(t·d) and the bio-drying pile entered its second thermophilic phase. From the 9th day to the 20th day, the water generation showed a decline. When the bio-drying process ended on the 20th day, the water generation was only 0.16 kg/(t·d). The mean degradation rate of organic matter peaked at 6.68 kg/(t·d) during the first thermophilic phase, while the one decreased to 2.29 kg/(t·d) during the cooling phase. After the bio-drying, the percent of easily degradable matter (hydrolyzable matter and lipid) in the total organic matter reduced from 49.91% to 37.94%. The variation in water generation in the bio-drying pile indicated that a large amount of metabolic water was generated between the 2nd day and the 9th day. The water generation could be attributed to the increase of microbial metabolism in the pile, as well as the high amount of organic matter that was degraded during this phase. Throughout the bio-drying process, the total water evaporation was 414.6 kg/t and the ratio of water generation to water evaporation was 1:6.7. Taken together, these findings indicate that much more water is evaporated than the generated.