A renewable bio-based thermosetting adhesive named tannin-furanic-formaldehyde (TFF) resin was synthesized using natural raw materials from crops and forest, such as furfuryl alcohol and bayberry tannin. The thermal properties of the adhesives were studied using differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and thermal gravimetric analysis (TGA). The structure of the TFF resin was characterized by electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared (FTIR) spectroscopy. The results indicated that TFF resin was easily prepared. Moreover, it showed an excellent modulus of elasticity (MOE) and thermal resistance. Moreover, the cross-linking reaction of bayberry tannin, furfuryl alcohol, and formaldehyde under acid condition was established.