To understand how extracellular polymer substance (EPS) used as an additive promotes methanogenesis, batch tests of methane production potential in anaerobic reactors with the addition of EPS and without EPS (control) were conducted. EPS markedly increased methane production during anaerobic digestion (25.0-36.5%) compared with the control reactor. EPS enriched functional microorganisms, including organisms belonging to the phyla Firmicutes, Actinobacteria, Synergistetes, and Chloroflexi, and optimized the community structure of microorganisms. Among them, 8.86% of the operational taxonomic units were from important hydrolysis and acidification phyla, which may be an important reason for the enhanced methanogenic capacity of anaerobic granular sludge (AnGS). Additionally, EPS increased the number of cytochrome c (c-Cyts) by 15.2-25.0% compared with the control reactor, accelerating the direct interspecies electron transfer between syntrophic bacteria (norank_f_Synergistaceae and Syntrophobacter increased by 58.2-102% and 115-151% compared with the control reactor, respectively) and methanogens (Methanosaeta), thus improving the anaerobic digestion performance. The results were validated by applying 16S function prediction. Interestingly, the average particle size, volatile suspended solids/total suspended solids, and EPS content of AnGS in the EPS reactors were approximately equal to the values in the control reactor during the anaerobic digestion, illustrating that EPS cannot affect the physicochemical properties of AnGS. These results suggest that EPS may be a conductive material in the anaerobic digestion process. Therefore, EPS used as an additive has great potential in the improvement of AD performance.