Theasinensin A (TSA) and theasinensin B (TSB), dimers of tea catechins produced during the processing of oolong tea and black tea, had superior inhibitory effects on α-glucosidase. However, the potential inhibitory mechanisms on α-glucosidase are still unclear. In the present study, TSA and TSB were chemically synthesized and purified, and their inhibitory effects on α-glucosidase and potential mechanisms were investigated. The results showed that TSA and TSB could inhibit the activity of α-glucosidase in a reversible and noncompetitive manner with IC50 values of 6.342 and 24.464 μg mL-1, respectively, which were much lower than that of acarbose. The fluorescence and circular dichroism spectra revealed that TSA and TSB could alter the microenvironment and the secondary structure of α-glucosidase, thereby decreasing the α-glucosidase activity. Molecular docking results indicated that both TSA and TSB had a strong binding affinity to α-glucosidase by hydrophobic interactions and hydrogen bonds. Moreover, the stronger inhibition of TSA on α-glucosidase might be related to the closer binding site to the active site pocket of α-glucosidase.