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Guavinoside B (GUB) is the main active substance in guava fruit and shows promising biological activities. In this study, the inhibitory activity and mechanism of GUB on α-glucosidase were studied by using spectroscopic techniques, kinetic analysis, and molecular docking. Results indicated that GUB possessed significant inhibition ability on α-glucosidase, which was about 10 times that of acarbose. The GUB was a mixed-type inhibitor, which suppressed the activity of α-glucosidase through a reversible process. Fluorescence analysis revealed that GUB quenched the fluorescence of α-glucosidase statically, the formation of GUB-α-glucosidase complex was a spontaneous and exothermic process, van der Waals forces, hydrogen bonding, and hydrophobic interaction were the predominant driving forces, only one single-binding site on α-glucosidase was involved in the binding process. GUB inserted into the hydrophobic pocket of α-glucosidase with 11 hydrogen bonds and two π-π stacking formed. The presence of GUB changed the microenvironment near the fluorescent amino acids of α-glucosidase, and the structure of α-glucosidase was slightly changed, eventually leading to the decrease of α-glucosidase activity. PRACTICAL APPLICATIONS: Diabetes mellitus (DM) is a worldwide chronic metabolic disease threatening human health seriously. Guava fruit is a popular fruit, and its extracts were reported to show many biological activities. GUB is the main benzophenone glycoside in guava fruits. However, the inhibitory activity and mechanism of its specific active compound GUB are still unclear. Studies have shown that GUB could reversibly inhibit the activity of α-glucosidase, and its inhibitory ability was about 10 times that of acarbose. The kinetics and mechanism of inhibition were revealed. These will facilitate the further research and application of guava fruit and GUB in functional and healthy foods against hyperglycinaemia or even DM., (© 2022 Wiley Periodicals LLC.)