Abstract: Coalbed methane (CBM) production involves a complex sequence of depressurization, desorption, diffusion, and seepage. However, conventional reservoir stimulation techniques often struggle to effectively enhance methane desorption and diffusion in coal seams. Coalbed Gas Bioengineering (CGB), an emerging production-enhancement technology, exploits anaerobic microbial fermentation to partially convert coal into methane and liquid organic compounds. This process exerts multiple synergistic effects, including enhanced methane desorption, improved diffusion, and increased reservoir permeability. Anaerobic fermentation within coal reservoirs is accompanied by complex biochemical reactions and associated heat release. Accurate quantification of this thermal output is therefore essential for elucidating the mechanisms underlying CBM enhancement. In this study, a self-developed calorimeter coupled with a simulated in-situ anaerobic fermentation system was employed to investigate the thermodynamic characteristics of microbial fermentation in long-flame coal. Continuous temperature monitoring was used to quantify the heat released during fermentation and to evaluate its influence on the methane adsorption capacity of coal. The results revealed a pronounced self-heating effect during in-situ anaerobic fermentation, characterized by four distinct temperature stages: slow heating, low-temperature stabilization, rapid heating, and high-temperature stabilization. During the initial stage of fermentation (days 1–12), long-flame coal was readily degraded by microorganisms. The detachment of macromolecular structures occurred without intense chemical reactions, and the energy absorbed during bond cleavage was approximately balanced by the energy released, resulting in only a slight temperature increase. Between days 13 and 16, the system reached a quasi-thermal equilibrium, with energy input and output remaining nearly balanced and the temperature stabilizing at approximately 31 °C. In the intermediate fermentation stage (days 17–23), metabolic activity was dominated by acidogenic and hydrogen-producing acetogenic bacteria. During this period, the concentrations of straight-chain alkanes, amines, organic acids, esters, and aromatic derivatives progressively increased. The conversion of complex coal macromolecules into these lower-molecular-weight compounds released substantial heat, leading to a marked rise in system temperature. As fermentation progressed into the peak methanogenic phase, organic acids (e.g., acetic and propionic acids) and alcohols (e.g., ethanol) were rapidly consumed by methanogens to produce methane. This process released a large amount of energy, resulting in a sharp temperature increase.The total heat generated during anaerobic coal fermentation was theoretically sufficient to increase the temperature of the solid–liquid system by approximately 6 °C. This temperature rise reduces the methane adsorption capacity of coal, thereby facilitating methane desorption and contributing to enhanced CBM production. Overall, this study provides direct experimental evidence for the thermodynamic effects of microbial activity in coalbed gas bioengineering and offers theoretical support for the application of CGB as an effective strategy to improve coalbed methane recovery.