The hydrothermal dewatering (HTD) of lignite results in noticeable variations in the low-temperature oxidation process. Consequently, this study was made on the gas release and temperature change characteristics to investigate the oxidation kinetics and mechanism of HTD coal samples. In this study, a lignite from Inner Mongolia in China was upgraded by HTD. N₂ adsorption, SEM, FT-IR, and chemical titration experiments were also carried out on raw and HTD coal samples to relate the physico-chemical structure properties with low-temperature oxidation characteristics. Results show that HTD coal samples have higher low-temperature oxidation activities and lower critical ignition temperatures compared with raw coal. According to the change in activation energy by kinetic analysis, the low-temperature oxidation process in the temperature range 35–140 °C could be divided into the stage I (oxygen adsorption stage) and stage II (accelerated oxidation stage). The correlation analysis indicates that the oxygen adsorption stage is controlled by the aliphatic and surface structures, while the accelerated oxidation stage is jointly affected by the competition of physico-chemical structures. The oxygen adsorption stage promotes the progress in accelerated oxidation stage.