In the present research, the effects of pumice on impact load resistance and microbial community structure in anaerobic granular sludge of up-flow anaerobic sludge bed reactors (UASB) were studied. Our results indicated that the addition of pumice was able to promote the degradation of organic matter, reduce the start-up time, and improve the impact load resistance of the UASB reactor. Compared with the control, the start-up time of the UASB reactor was reduced by 12 days. When the organic load gradually increased from 9.84 kg COD/(m³·d) to 33.28 kg COD/(m³·d), the average COD removal rate in the reactor containing pumices increased by 10.53% as compared to control. High-throughput sequencing showed that the addition of pumices provided a favorable environment for microbial aggregation, and increased the richness and diversity of microorganisms in the granular sludge. The community structure of methanogens indicated that, in the presence of pumices and under the impact of organic load, the dominant methanogens were Methanosaeta instead of Methanbacterium.