<p>Most of the phenolic compounds in black highland barley (BHB) exist in complex, insoluble bound forms with low bioavailability. Various food processing methods, including fermentation, have been traditionally used to enhance bioaccessibility and improve the functional properties of food products. This study aimed to investigate the alterations and relationships between the content and composition of phenolic compounds in different forms, hydrolytic enzyme activities, and in vitro antioxidant activities during solid-state fermentation (SSF) of BHB by <i>Bacillus subtilis</i>. The results revealed a significant increase in total phenolic content (TPC) and total flavonoid content (TFC) during fermentation, particularly in the free phenolic content. The composition of phenolic compounds in both free and bound fractions of BHB was altered by fermentation. The o-coumaric acid (13.23&#xa0;mg/100&#xa0;g), 2,4-dihydroxybenzoic acid (267.43&#xa0;mg/100&#xa0;g) and tectorigenin (64.37&#xa0;mg/100&#xa0;g) were released in the free fraction after fermentation. The bound polyphenol content showed strong correlations with cellulase, β-glucosidase, and protease activities, while free o-coumaric acid and free&#xa0;<i>p</i>-hydroxybenzaldehyde showed strong correlations with α-amylase. The antioxidant activities of phenolics in fermented BHB were significantly higher than in unfermented BHB, with free phenolic content positively correlated with antioxidant capacity (<i>p</i> &lt; 0.05). Among the free phenolics, protocatechuic acid, gallic acid, and 2,4-dihydroxybenzoic acid were the main contributors to FRAP and ABTS⁺• scavenging abilities, while vanillic acid and ferulic acid in the free fraction were the primary contributors to DPPH• scavenging. This study highlights the potential for producing fermented BHB as a functional ingredient with enhanced bioactivity for health promotion.</p> Graphical Abstract <p></p>