The construction of fully bio-based epoxy resins (EP) has been of particular interest in both academia and industrial circles for years; among these, epoxidized soybean oil (ESO) derived thermosets have received the most attention, but they usually exhibit poor performance due to their flexible fatty chains. Herein, tannic acid (TA), with its great degree of functionality and massive aromatic structures, was chosen as the multi-phenol curing agent for ESO to prepare fully bio-based EP thermosets with a high relaxation temperature and satisfactory mechanical properties. As a natural 2-substituted imidazole-containing substance, histidine (H) was used as the curing accelerator under moderate curing conditions (120-180 °C). This EP system showed high curing activity and a good curing degree while operating. The cured thermosets were found to be thermally stable (T 5% > 270 °C) and displayed a high relaxation temperature (77 °C) with a tensile strength of 23 MPa. Preliminary adhesion tests showed that the cured product exhibited a high lap-shear strength of about 19 MPa in adhesion failure mode. Taking these advantages into account, this kind of fully bio-based EP could introduce more chances for versatile applications, such as being used in structural materials and construction adhesives.