The polysaccharide (JAP) acts as a prebiotic in ruminants. This study investigated the effects of dietary JAP supplementation on growth performance, meat traits, rumen microbes, and metabolome of fattening lambs. A total of 72 healthy crossbred ram lambs (Small Tail Han sheep × Black Bone sheep), aged three months (body weight = 26.21 ± 2.14 kg), were randomly assigned to 12 pens. The pens were evenly divided into two groups: one receiving a basal diet (the CON group) and the other receiving a basal diet supplemented with 0.5% JAP (the JAP group). Compared to the CON group, the JAP group exhibited higher final body weight ( = 0.028), average daily gain (ADG) ( = 0.013), and carcass weight ( = 0.004), as well as a significantly lower feed conversion ratio (FCR) ( = 0.015). Compared to the CON group, the concentration of ruminal propionic acid in the JAP group increased ( < 0.001), while that of ammonia nitrogen (NH-N) decreased ( < 0.001). Additionally, the JAP group showed increased ( = 0.024) serum total protein content and decreased ( = 0.005) serum triglyceride content when compared with CON group. Furthermore, higher levels of tryptophan ( = 0.040) and erucic acid (C22:1n9) ( = 0.010) were detected in the longissimus dorsi muscle of the JAP group compared to the CON group. JAP supplementation altered the rumen microbiota composition at the genus level. This included increases in the relative abundances of ( < 0.001), ( = 0.001), ( = 0.003), ( < 0.001), ( < 0.001), and ( < 0.001). There were also decreases in the relative abundances of ( = 0.014), ( < 0.001), ( < 0.001), ( = 0.001), ( < 0.001), ( < 0.001), ( = 0.002), ( = 0.032), ( = 0.028), ( < 0.001), ( = 0.003), ( < 0.001), and ( < 0.001). A total of 23 differential metabolites were identified, enriched in 11 metabolic pathways, particularly those related to protein and lipid metabolism between the two groups. In conclusion, dietary JAP affected rumen microbes and metabolites, influenced protein and fat metabolism, improved the fatty acid and amino acid composition in meat, and enhanced the growth and production performance of fattening lambs.