Microplastics (MPs) are defined as plastic debris with particle size smaller than 5 mm, which have been frequently detected in environments. In this study, earthworms (Eisenia foetida) were exposed to three different sized polystyrene (PS; 0.1,10 and 100 mu m) at 10 and 100 mg per kg soil for 21 days. We examined the contents of metals (Cu, Zn, Ni and Pb) and gut microbial communities in earthworms exposed to MPs. Results showed that MPs reduced the accumulation of Ni and Pb in earthworms on the 21st day. The composition of gut bacterial communities was altered in earthworms exposed to MPs, especially 10 mu m MPs, featuring a higher relative abundance of Proteobacteria (44.5%) and Bacteroidetes (27.1%) than the control group. Additionally, gut microorganisms including genus Paenibacillus and Achromobacter in earthworms were expected to be potential biomarkers for Cu and Zn. The microbial community networks showed that MPs affected bacterial community connections by size effect, and MPs with smaller size increased the complexity of earthworm microbial community networks. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that metabolism was the main difference in the bacterial communities in different treatments, especially carbohydrate metabolism and amino acid metabolism, which was relevant to the survival and growth of bacteria. This study provides insight into the environmental risks of MPs on terrestrial organisms.