Anaerobic digestion (AD) is a promising process for organic waste treatment to achieve both the waste reduction and energy generation.However, despite a continuously increasing interest and popularity on AD, the process instability under high organic loading rate (OLR) is a concern.Microorganisms are the core of the anaerobic digesters;the understanding of the microbial community is crucial for improving efficiency and process stability of anaerobic digesters.To investigate the linkage between process stability and microbial community, and describe the microbial mechanisms of system instability, the study introduced OLR disturbances into a mesophilic anaerobic digester treating food waste (FW) to induce stable and deteriorative phases, during which physicochemical analysis along with the pyrosequencing microbial technique were performed to monitor process parameters and microbial population of each phase, respectively.Results show that the relative abundance of acid-producing bacteria and syntrophic volatile fatty acid (VFA) oxidizers increased dramatically at deteriorative phase, while the dominant methanogens did not shift from acetoclastic to hydrogenotrophic groups.The mismatching between bacteria and methanogens may partially be responsible for the process deterioration.Moreover, the succession of predominant hydrogenotrophic methanogens reduced the consumption efficiency of hydrogen;meanwhile, the dominant Methanosaeta with low acetate degradation rate, and the increase of inhibitors concentrations further decreased its activity, which may be the other causes for the process failure.These results improve the understanding of the microbial mechanisms of process instability, and provide theoretical basis for the efficient and stable operation of anaerobic digester treating FW.