Astragalus polysaccharide possesses promising potential as a natural non-antibiotic growth promoter for ruminant production. This study investigated the effects of dietary supplementation with coated Astragalus polysaccharide (CAPS) on growth performance, nutrient digestibility, rumen fermentation, serum metabolites, and antioxidant capacity in Angus beef cattle. A total of 60 late-fattening Angus bulls (Body weight: 650 ± 29.6 kg) were randomly divided into four treatment groups. Bulls in the four treatment groups were fed a basal diet supplemented with 0, 0.9, 1.8, and 2.7 g of CAPS (Control, LCAPS, MCAPS, and HCAPS, respectively) per kg of dietary dry matter (DM) for 60 days. Increasing dietary CAPS supplementation linearly increased average daily gain (ADG) and nutrient digestibility while quadratically decreasing feed-to-gain ratio (F/G), with MCAPS treatment resulting in the highest ADG and nutrient digestibility and the lowest F/G. MCAPS and HCAPS treatments stimulated ruminal total short-chain fatty acid production and shifted the fermentation pattern toward propionate formation. Feeding CAPS linearly increased the abundance of Rumincoccus albus, Rumincoccus flavefaciens, Ruminobacer amylophilus, and Prevotella ruminicola and the activity of carboxymethyl-cellulase, laccase, α-amylase, and protease. Blood urea nitrogen and serum cholesterol concentrations were decreased linearly with CAPS supplementation. Moreover, the serum oxidative stress parameters showed that 1.8 and 2.7 g/kg DM of CAPS addition increased total superoxide dismutase, glutathione peroxidase, and catalase activities and decreased malondialdehyde contents. These findings demonstrate that supplementing the diet with 1.8 g/kg DM of CAPS optimally enhanced growth performance, rumen function, metabolic efficiency, and serum antioxidant status in Angus bulls.