The sheep rumen microbial community plays an important role in animal performance and the environment. Few studies have paid close attention to the impact of different levels of dietary nutrition on rumen microbial populations. A total of 112 healthy Tan sheep were selected and randomly allotted to one of four dietary treatments (groups I, II, III, and IV). Each treatment included four replicated pens with seven sheep each for a total of 28 sheep per treatment. The sheep were fed four diets with nutrient levels that were 84, 96, 108, or 120% of the recommendation. In this study, a next-generation sequencing strategy and quantitative real-time PCR analysis were applied to investigate changes in whole ruminal bacteria with increased dietary energy and protein levels. The study observed 133 genera belonging to 16 phyla distributed throughout the rumen samples, with Firmicutes and Bacteroidetes predominating. Additionally, the higher nutritional dietary level linearly increased (P < 0.05) the number of Bacteroidetes and Proteobacteria but linearly decreased (P < 0.05) the Firmicutes richness. At the species level, the abundance of Prevotella ruminicola, Ruminococcus flavefaciens, and Succinivibrio dextrinosolvens linearly increased (P < 0.05), whereas the abundance of Selenomonas ruminantium and Veillonella parvula did not (P > 0.05). Furthermore, we predicted the potential functions of rumen bacteria. In particular, the relative abundances of the genes related to carbohydrates were overrepresented, and the genes involved in amino acid metabolism linearly increased (P < 0.05). These findings provide the first deep insights into the rumen microbial composition and the targeted improvement of dietary protein and energy use efficiency in Tan sheep.