A terahertz biosensor based on Fano resonance is proposed. The sensor is composed of a graphene microdisk trimer structure. The finite element method is applied to investigate the transmission characteristics of the structure. Simulation results exhibit that the graphene trimer structure can excite Fano resonance. The structure yields a high sensitivity of 2 THz/RIU and a figure of merit （FOM） of 8 in the THz spectrum； the graphene chemical potential can be used to adjust the resonance peak. The band realizes the active regulation of the resonance peak. This structure provides a design idea for the ultra-sensitive terahertz sensor based on metamaterials， and has potential application value in the sensing and detection of materials with micro-nano thickness.