A broadly targeted metabolomics approach based on UPLC-MS/MS was employed to investigate the changes in chemical composition and in vitro activity of highland barley Monascus tea decoction before and after simulated digestion. The characteristic metabolites of the tea decoction before and after in vitro-simulated digestion were identified, and the in vitro antioxidant and enzyme inhibitory activities of the tea decoction were further analyzed. The study detected 1431 metabolites, including amino acids and their derivatives, alkaloids, organic acids, nucleotides and their derivatives, lipids, terpenoids, and phenolic acids. A total of 136 differential compounds were identified, primarily distributed in amino acids and their derivatives, alkaloids, organic acids, phenolics, and lipids. in vitro-simulated digestion significantly increased the content of amino acids, alkaloids, lipids, and phenolics in the tea. The differential metabolic compounds were primarily assigned to 20 metabolic pathways, mainly involving the metabolism of amino acids, nucleotides, carbohydrates, fatty acids, and other compounds. Additionally, after simulated digestion in vitro, the comprehensive antioxidant index (60.53%), α-glucosidase inhibitory activity (54.35%), and pancreatic lipase inhibitory activity (4.06%) was significantly improved. The highland barley Monascus tea decoction showed potential hypoglycemic and hypolipidemic efficacy. This study can provide a theoretical basis for the high-value utilization of highland barley and the development of healthy grain tea.