Earthworm guts are thought as a potential reservoir of microbial diversity. Low concentrations of arsenic are widespread in soil and the effect of arsenic contaminant on earthworm gut microbiota has been confirmed. However, shifts of arsenic biotransformation in different earthworm gut are largely unknown. In this study, four species of earthworms were exposed to arsenic-contaminated soils to explore the difference and universality of the effects of low concentrations of arsenic (i.e., concentrations of 5, 15, and 25 mg/kg) on the gut microbiota characteristics using Illumina high-throughput sequencing, and to examine the relationships between arsenic enrichments, arsenic species and arsenic biotransformation genes in different earthworm gut. Our results demonstrated that significant arsenic bioaccumulations in all earthworm body tissues were observed. Eisenia Andrei had the highest arsenic bioaccumulation, in which the bioconcentration factor of arsenic was 1.93, followed by Metaphire californica (0.80), Metaphire vulgaris (0.78), and Amynthas hupeiensis (0.52). Inorganic arsenic (As (V) and As (III)) was predominant in earthworm body tissues and guts, where the percentage of As (III) was more than 80%. A few of organic arsenic was also observed in E. Andrei and A. hupeiensis body tissues. Gut microbial communities in four species of earthworms were dominated by Proteobacteria (22.7%), Firmicutes (25.9%), and Actinobacteria (28.0%) at the phylum level, and were significantly different from those in the surrounding soil. In addition, a total of 17 ABGs were quantified in soil and earthworm gut samples by high throughput quantitative polymerase chain reaction (HT-qPCR), a higher relative abundance of genes involved in As (V) reduction and arsenic transport, and a lower abundance of genes involved in arsenic methylation and demethylation were observed in all earthworm gut samples. The earthworm gut can be a reservoir of microbes with the capability of reducing As (V) and extruding As (III), but with little methylation and demethylation of arsenic, suggesting that ABGs played important roles in the biotransformation and bioaccumulation of arsenic. Moreover, low concentrations of arsenic did not significantly alter the survival and growth of earthworms, but it could disturb the bacterial community of earthworm gut. Changes of microbial community in the earthworm gut were mainly influenced by earthworm species and arsenic contamination. In short, this study suggested that earthworm gut was an important hotspot for the microbe-mediated arsenic biotransformation, and the findings of this study could broaden our understanding of the biogeochemical behavior of arsenic in the gut of soil animals.