Abstract: The early diagnosis of brain tumors remains challenging, particularly in terms of non-invasive, rapid, and highly sensitive detection techniques. In this paper a ring-type surface plasmon resonance terahertz microstructure optical fiber biosensor was optimal designed for precise detection and differentiation between healthy and tumorous brain tissues. The sensor employs an external sensing mechanism, with its outer cladding coated with PVDF as the SPR excitation layer, enabling strong coupling between PVDF and target bioanalytes. The structural parameters of the fiber were optimized using the full-vector finite element method, and the sensing performance was systematically analyzed.Experimental results demonstrate the biosensor achieves a maximum amplitude sensitivity of −125.95 RIU⁻¹, a superior figure of merit of 71.87 RIU⁻¹, and refractive index resolution on the order of 10⁻⁷ in the effective refractive index (RI) range of 1.38~1.51. In addition, the tolerance analysis results show that the sensor can maintain stable performance within a ±2% fabrication tolerance range, demonstrating its good fabrication feasibility. The optimized biosensor holds substantial potential for early-stage brain tumor diagnosis in biomedical field applications, offering high sensitivity and resolution to advance clinical detection and improve patient outcomes.