Background <p>Yaks are important livestock species on the Qinghai-Tibet Plateau (QTP), but their productivity is constrained by the harsh alpine environment and the seasonal scarcity of forage resources. Improving feed efficiency has become the key to the sustainable development of Plateau Yak breeding industry. Phytosterols have shown a good application prospect in ruminant feed efficiency improvement, but the impact on yaks is not clear. This study explored the effects of dietary phytosterols supplementation on growth performance, nutrient digestibility, rumen microbial community structure and metabolic function of yaks fed in a barn.</p> Results <p>Twenty-eight 1.5-year-old male yaks (137.10 ± 8.70&#xa0;kg) were randomly divided into two groups after 28 days of pre feeding period: the control group (Con, <i>n</i> = 14) was fed with basic diet, and the experimental group (PS, <i>n</i> = 14) was added with 200&#xa0;mg/kg phytosterols in concentrate. The formal test period was 60 days. Compared with Con group, PS group significantly increased average daily gain (<i>P</i> = 0.001), apparent digestibility of crude protein (<i>P</i> = 0.036) and neutral detergent fiber (<i>P</i> = 0.006), and reduced feed conversion rate (<i>P</i> = 0.002). The rumen fermentation mode of PS group changed, the proportion of propionate increased significantly (<i>P</i> = 0.001), while the proportion of acetate (<i>P</i> = 0.006), acetate to propionate ratio (<i>P</i> = 0.001) and lactate (<i>P</i> = 0.035) concentration decreased significantly. Phytosterols significantly changed the structure of rumen microbial community and significantly increased the copy number of rumen bacteria (<i>P</i> &lt; 0.05). Metatranscriptomic analysis showed that PS group significantly increased the relative abundance of key bacteria, including <i>Succiniclasticum</i>, <i>Faecalibacterium</i>, <i>Ruminococcus</i>, <i>Butyrivibrio</i>, and <i>Alistipes</i> (<i>P</i> &lt; 0.05). Microbial co-occurrence network analysis revealed that the number of edges, average degree and modular index of microbial community structure network in PS group increased. Rumen function analysis showed that glycolysis / gluconeogenesis and propionate metabolism were significantly enhanced in PS group (<i>P</i> &lt; 0.05), and the expression of key enzymes such as hexokinase and fumarate hydratase were significantly enhanced (<i>P</i> &lt; 0.05). Metabolomic analysis revealed that phosphoenolpyruvic acid, fumarate and methylmalonyl-CoA were significantly accumulated in the rumen of PS group (<i>P</i> &lt; 0.05). Pathway analysis showed that the pathway impact value of propionate metabolism and glycolysis / gluconeogenesis was greater than 0.1.</p> Conclusions <p>Phytosterols (200&#xa0;mg/kg of concentrate) can effectively improve the feed efficiency of yaks by regulating the structure and function of rumen microorganisms and altering the fermentation patterns.</p> Graphical Abstract <p></p>