Responses of diverse gut microbiota affect the host's nutrient metabolism and disease immunity. However, giant pandas (Ailuropoda melanoleuca) have a low diversity of gut microbiota, and the reasons for this low diversity and its impact are poorly understood. In this study, we aimed to investigate the effects of diet on the diversity of gut microbiota in giant pandas. To analyze the functional genes of the gut microbiota, we employed PICRUSt (phylogenetic investigation of communities by reconstruction of unobserved states), a computational method for predicting the functional composition of genomes. A multispecies comparison was conducted, involving 66 individuals from 34 species including giant panda. Our comparative analysis revealed that a high-fiber diet significantly reduced the diversity of gut microbiota in giant pandas. Furthermore, we observed that the metabolic function of giant pandas, particularly in terms of energy families and amino acid metabolism, was significantly lower than in other species. During the high-fiber diet, the gut microbiota of giant pandas exhibited a greater abundance of genes involved in carbohydrate metabolism, amino acid metabolism, and transport functions. These findings suggest that a high-fiber bamboo (Poaceae) diet is associated with lower diversity and altered metabolic function of gut microbiota in giant pandas. Therefore, the gut microbiota of giant pandas undergoes functional adaptive changes in response to the consumption of a high-fiber bamboo diet.