The development of an effective biocathode with high catalytic ability and dense biomass is a major challenge for the industrial applications of electromethanogenesis (EM) process. In our previous study, intact anaerobic granular sludge (AnGS) biocathode and EM hybrid system (AnGS-EM) showed superior ability and stability when treating raw biogas, but its maximum CO2-to-CH4 conversion potential and the response to different operating conditions are still unknown. Herein, we optimized the performance of the AnGS-EM system and explored its maximum CH4 production capacity. The AnGS-EM system achieved a maximum methane production rate of 202.15 L CH4/m2catproj/d, which is over 3 times higher than the maximum value reported so far. Within a certain range, the methane production rate increased with the buffer concentration, applied voltage, and bicarbonate concentration. Excessive applied voltage and carbonate concentration not only led to resource waste but also inhibited methanogen performance. The AnGS biocathode could withstand oxygen exposure for 24 h, the acidic (pH of 5.5), and alkaline conditions (pH over 9). Illumina sequencing results showed that hydrogenotrophic methanogen (especially Methanobacterium) were dominant. This work using AnGS as biocathode for CH4 synthesis offers insight into the development of scalable, efficient, and cost-effective biocathode for biofuels and value-added chemicals production.