This experiment aimed to investigate the effects of dietary supplementation with Glycyrrhiza polysaccharides （GCPs） on the growth performance， serum indicators， apparent nutrient digestibility， rumen fermentation parameters and microbial communities in fattening beef cattle. Forty Simmental fattening bulls with good growth status and an average body weight of 617.70±18.85 kg were selected as experimental animals and randomly divided into four groups. The experimental groups received the basal diet supplemented with 0 mg·kg^-1 BW （control group， G0）， 15 mg·kg^-1 BW （low-dose， G15）， 30 mg·kg^-1 BW （medium-dose， G30）， or 45 mg·kg^-1 BW （high-dose， G45） of GCPs， respectively. The trial included a 10-day adaptation period followed by a 90-day feeding period. The body weight were measured at 0， 45，90 d of the feeding period， daily feed intake was recorded to determine average dry matter intake and feed-to-gain ratio. At 45 and 90 d of the feeding period， 10 mL of blood sample was collected from each bull to assess serum biochemical， antioxidant and immune indicators. At the end of the trial， 300 g of feces sample was collected from each bull to evaluate nutrient apparent digestibility， and 50 mL of rumen fluid was collected to analyze rumen fermentation parameters and microbial related indicators. Compared with the G0 group： 1） During the 0-45 d and 0-90 d periods， the average daily gain and average dry matter intake of the G30 and G45 groups were significantly increased （P<0.05）； in the 0-90 d period， the feed conversion ratio of the G30 and G45 groups decreased significantly （P<0.05）， while the crude fat digestibility increased significantly （P<0.05）； 2） The serum aspartate aminotransferase level and alanine aminotransferase level of the G30 group were significantly decreased （P<0.05）； at 45 d， the serum levels of superoxide dismutase and interleukin-10 in the experimental groups increased significantly （P<0.05）； 3） The acetic acid proportion of the G15 and G30 groups was significantly decreased （P<0.05）， the butyric acid proportion of the G30 group was significantly increased （P<0.05）； 4） The Chao1 richness index of the rumen microbial community in the G30 group was significantly increased （P<0.05）， along with structural changes in microbial composition. In the G30 group， the relative abundance of Bacteroidota， Pseudomonadota， Fibrobacterota， Prevotella and Fibrobacter showed a significant upward trend， while the relative abundance of Bacillota showed a significant downward trend （P<0.05）； the differential metabolic pathways of rumen microorganisms in the G15 group were mainly enriched in Synthesis and Degradation of ketonebodies， and nitrotoluene degradation. In the G30 group， these pathways were mainly linked to Amino acid metabolism， and Fatty acid biosynthesis， while the G45 group showed enrichment in C5-branched dibasic acid metabolism. In summary， dietary supplementation of Glycyrrhiza polysaccharides enhanced the growth performance， antioxidant capacity， immune function， and nutrient digestibility of Simmental fattening beef cattle， while optimizing rumen fermentation patterns and promoting beneficial rumen microorganisms. Under the conditions of this experimental study， the optimal addition amount of GCPs was 30 mg·kg^-1 BW.