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A highly sensitive and tunable biosensor at terahertz (THz) frequencies is proposed by using a composite structure of graphene and one-dimensional photonic crystal. The theoretical analysis and simulation results show that the sensitivity and figure-of-merit of the biosensor can reach 517.9°/RIU and 222.9RIU- 1, respectively. The high sensitivity of the biosensor originates from the excitation of optical Tamm states caused by graphene-based distributed Bragg reflector. By varying the voltage on graphene, we can electrically control the sensitivity and figure-of-merit of the biosensor. Furthermore, we have shown that the biosensor has a wide range (1.40–1.70) for measuring the refractive index of the sensing medium. We believe the biosensor has a good application prospect in the field of biological detection. Keywords: Biosensor, Terahertz, Graphene, Photonic crystals, Optical Tamm states