[Objectives]  To investigate adaptative changes in the intestinal microbial community of Thitarodes xiaojinensis larvae following infection by Ophiocordyceps sinensis with the goal of predicting dominant microorganisms in infected individuals based on their function, and explain the nutrient supply mechanism underlying the increased abundance of certain microorganisms in cultivated Cordyceps sinensis. [Methods]  Healthy eggs and intestinal tissues from 1^st and 5^th instar larvae, pupae and adults, were selected for use as controls and intestinal tissues from mature 5^th instar larvae infected with Ophiocordyceps sinensis were collected for analyses. 16S rDNA sequencing was used to evaluate differences in microbial community structure and diversity between infected and non-infected intestinal contents. [Results]  In healthy intestinal tissues, bacterial abundance gradually decreased with age, from 1 391 species in first instar larvae, to 305 species in pupae. In healthy tissues, Proteobacteria were the dominant flora with only a few Firmicute species present. However, in infected larvae, the biological richness, diversity, and evenness of intestinal microorganisms decreased. The Shannon index, Simpson diversity index, and Pielou's evenness index of infected larvae were significantly different to those of the control group (P=0.05), and the Firmicutes were the dominant intestinal flora. Increases in Lactobacillus and Carnobacterium species were significant and pronounced. Analyses of microbial metabolic pathways after fungal infection revealed that energy metabolism pathways (retinol, inositol phosphate, fructose and mannose) were significantly up-regulated (P<0.001). Pyruvate fermentation, pentose phosphate, and glycolysis pathways were also up-regulated by 8.44-fold, 4.19-fold and 2.90-fold, respectively. Host digestive capacity was verified, confirming that intestinal digestive function was significantly improved. Trypsin, sucrase, and lipase increased by (13 993±665.00), (0.52±0.06) and (0.04±0.01) U/mL, respectively. The increase in amylase was especially significant; enzyme activity increased by (0.68±0.01) U/mL (P<0.001). [Conclusion]  O. sinensis infection changes the dominant intestinal flora of T. xiaojinensis from Proteobacteria to Firmicutes. Firmicutes participate in polysaccharide degradation, promoting nutrient absorption and utilization by the host.