Abstract: To address the limitations of single-strain fermentation of tea, specifically its single metabolic pathway and limited flavor enhancement, this study employed co-liquid-state fermentation of crude dark tea using Aspergillus cristatus G-Y and Bacillus subtilis SO-2, with unfermented tea broth as the control (CK group). The synergistic effects of the dual strains on the regulatory mechanisms of soluble sugars, tea pigments, phenolic compounds, antioxidant activity, and flavor substances in tea broth were systematically investigated. The results demonstrated that co-fermentation significantly reduced polysaccharide content (0.61 mg/mL, P<0.05) and promoted the accumulation of theabrownins (positively correlated with Δa value). In terms of polyphenol metabolism, single-strain fermentation with B. subtilis reduced the levels of total polyphenols and total flavonoids (by 11.96% and 18.71%, respectively), and single-strain fermentation with A. cristatus primarily drove the degradation of esterified catechins. Importantly, co-fermentation retained the degradation advantage of A. cristatus single fermentation on esterified catechins (reduction>90%), and further regulated the polyphenol transformation pathway through microbial synergy, promoting the accumulation of gallic acid, free catechins, and other compounds, and significantly enhancing antioxidant activity (DPPH IC₅₀=0.0315 mg/mL; ABTS IC₅₀=0.0720 mg/mL, P<0.05). Flavor analysis revealed that the dual-strain interaction coordinated amino acid metabolism, leading to a balanced accumulation of sweetness-associated amino acids (141.59 mg/L) and bitterness-associated amino acids (129.01 mg/L). Moreover, through a coupled pathway of "glucosinolate hydrolysis (A. cristatus)-esterification (B. subtilis)," key aroma compounds such as benzyl isothiocyanate were substantially enhanced (ROAV=50.77, up to 49-fold higher than in single-strain fermentations). The total volatile metabolites increased by 6.18% compared to the control. Sensory evaluation confirmed that the co-fermentation group achieved the highest scores in aroma and taste, highlighting the synergistic effect of the dual strains on flavor balance. This study demonstrates that co-fermentation with A. cristatus and B. subtilis effectively modulates the transformation of bioactive compounds and the synthesis of key flavor substances through complementary metabolic pathways, significantly improving the overall quality of tea broth.