This paper designs a D-shaped microstructured fiber optic biosensing model based on surface plasmon resonance of graphene, which is dedicated to the terahertz frequency band. Improving optical fiber sensing performance by adjusting graphene thickness and graphene chemical potential to increase peak spectral loss. The optimization results show that the resonance peak decreases with increasing excitation layer thickness and graphene chemical potential. Moreover, the higher the graphene chemical potential, the more the resonance spectrum is shifted towards the higher frequency band. The maximum frequency sensitivity is 1300 GHz/RIU over a refractive index sensing range of 1.1-1.5, corresponding to a quality factor of 1.0× 10 4 RIU. In conclusion, the proposed D-type microstructured optical fiber has a novel and ingenious structure with high detection accuracy. With graphene as the excitation medium, the microstructured optical fiber operating band is successfully extended to the terahertz band, opening up a broader application prospect for the microstructured optical fiber structure in the THz sensing field. © 2023 IEEE.