<p>This study aimed to investigate the changes in active ingredients, nutrient contents and microbial communities during two-stage fermentation of Kudzu by-products (leaves and stems) and evaluate their effects as feed supplement for Hu lamb. The results showed that the fermentation process significantly enhanced the nutritional value of Kudzu by-products, with the crude protein and fat contents increasing from 13.51 to 16.37% and 1.78 to 6.55%, respectively. Furthermore, enhanced growth of probiotic microorganisms was observed. In particular, <i>Lactobacillus</i>, <i>Pediococcus</i>, <i>Bacillus</i>, <i>Issatchenkia</i> and <i>Nakaseomyces</i> were found to be present in relatively high abundance in the fermented Kudzu by-products. To gain further insight, non-targeted metabolomics was employed to analyze differentially expressed metabolites (DEMs). A total of 1209 DEMs were identified, of which 624 were up-regulated and 585 were down-regulated. Specifically, increased levels of metabolites, such as sucralose, N-acetyl-L- methionine, L-methionine, ganoderic acid A, D-threitol, vitamin B3, ganoderic acid C2, cucurbitacin B and 3-phenyllactic, which are known to positively influence the growth performance and health of animals, were noted. The dietary supplementation of fermented Kudzu by-product significantly improved the growth performance and immune function of the Hu lamb. This study suggest that fermentation with <i>Aspergillus niger</i> and <i>Lactobacillus plantarum</i> can significantly improve the feeding value of Kudzu by-products, and also providing new insights into their potential as an animal feeding resource.</p> Graphical abstract <p></p>