Abstract: To address the underutilization of blueberry pomace and the limitations of single-substrate fermentation, this study developed a novel composite substrate composed of oats and blueberry pomace. Ganoderma lucidum was employed in solid-state fermentation to enhance the production of bioactive compounds. A combination of single-factor experiments and Box–Behnken response surface methodology was used to systematically optimize material loading, inoculum size, and potassium dihydrogen phosphate supplementation for maximal triterpenoid synthesis. Untargeted metabolomics was conducted to identify differential metabolites before and after fermentation. The optimal fermentation conditions were determined to be a total material loading of 29 g, an inoculum size of 13% (v/w), and potassium dihydrogen phosphate addition of 0.1% (w/w), resulting in a triterpenoid content of 0.35%—a 1.3-fold increase compared to the unfermented substrate. The fermented product exhibited significantly enhanced antioxidant activity, with IC₅₀ values for DPPH, hydroxyl, and superoxide anion radicals reduced by 77.54%, 66.90%, and 56.92%, respectively (P<0.05). Furthermore, the inhibition rates against α-amylase and α-glucosidase increased by 1.10-to 1.57-fold relative to the unfermented substrate. Metabolomic analysis identified 106 differential metabolites across 18 categories, with 70 significantly upregulated, including nine terpenoids. KEGG pathway analysis indicated the activation of secondary metabolic pathways, particularly in terpenoid biosynthesis. These findings demonstrate that Ganoderma lucidum fermentation effectively enriches hypoglycemic bioactive components, offering a theoretical foundation for the high-value utilization of agricultural products and processing by-products.