In this study, sulfate was employed as an external electron acceptor for enhancing the degradation of propionate in a thermophilic anaerobic membrane reactor (AnMBR). The organic loading rate (OLR) was increased gradually from the initial 3.9 kg-COD/m3d to the inhibiting OLR of 14.6 kg-COD/m3d. Feeding was stopped for 98 days but the process did not recover until 500 mg/L of sulfate was added into the AnMBR. After that, the enhanced propionate degradation was achieved up to an OLR of 15 kg-COD/m3d with a reduced sulfate addition of 300 mg/L. However, the thermodynamic calculation indicated that the syntrophic propionic acid degradation, coupled with methanogenesis, was unfavorable with a △G of +3 kJ/mol under the enhanced conditions. Conversely, the utilization of propionic acid by sulfate reduction bacterial (SRB) would be more favourable by having a much lower △G of −180 kJ/mol. The hydrogen conversion was presumed to go through the methanogenesis pathway according to the thermodynamic results. The mechanism of the propionic and hydrogen metabolism was supported as well by comparing the microbial communities with and without sulfate addition. As a result, the role of the sulfate enhancing propionic degradation can be concluded by combining the process performance, thermodynamic, and microbiology results.