Background <p>The purpose of this study was to investigate the effects of changes in rumen microbial adaptability on rumen fermentation and nutrient digestion in Horqin beef cattle during different seasons of grazing and supplementary feeding. Four healthy female Horqin beef cattle were selected as experimental animals for grazing and supplementary feeding. The feed, feces, and rumen fluid were collected in spring (March), summer (June), autumn (September), and winter (December) to determine nutrient digestibility, rumen fermentation parameters and microflora. The microflora was sequenced with 16S rRNA.</p> Results <p>The results revealed that the rumen volatile fatty acid, ammonia nitrogen and microbial protein concentrations in summer were significantly higher than those in the other seasons (<i>P</i> &lt; 0.05). The digestibility of dry matter, organic matter, crude protein and ether extracts decreased significantly in autumn and winter (<i>P</i> &lt; 0.05), especially in winter. Compared with autumn, the rumen bacterial diversity of beef cattle increased significantly in spring (<i>P</i> &lt; 0.05). The results of the rumen microflora revealed that <i>Prevotella</i> was the most dominant bacteria in the rumen of beef cattle in summer, and <i>Rikenellaceae_RC9_gut_group</i> was the most dominant bacteria in the other seasons. The low quality of forage in spring, autumn and winter resulted in an increase in the relative abundance of <i>Rikenellaceae_RC9_gut_group</i>, <i>NK4A214_group</i>, and <i>Christensenellaceae_R-7_group</i>, etc., which are associated with fibre decomposition. In addition, nucleotide metabolism and glycan biosynthesis and metabolism increased significantly in bacteria in the summer (<i>P</i> &lt; 0.05).</p> Conclusions <p>The results revealed the dynamic adaptability of the rumen microbiota to seasonal and dietary changes and provide references for the grazing and supplementary feeding of Horqin beef cattle in different seasons.</p>