Exosomes, as critical biomarkers for cancer diagnosis and metastasis monitoring, are limited by current detection techniques due to low sensitivity and refractive index (RI) detection ranges. To address this, this study proposes a germanium-doped photonic crystal fiber (PCF) biosensor integrated with surface plasmon resonance (SPR) technology. The sensor features a D-shaped structure with three layers of periodic air holes, germanium-doped core capillaries, and a gold film coating, which expands the effective RI detection range to 1.00-1.45 RIU and achieves a maximum wavelength sensitivity of 30,000 nm/RIU at 1500 nm. Simulation and optimization results demonstrate its ability to distinguish normal exosomes (1.35-1.38 RIU), tumor exosomes (1.39-1.42 RIU), and pro-metastatic exosomes (PMEs, 1.43-1.45 RIU) by monitoring resonance wavelength shifts, with significantly enhanced sensitivity in the high RI range (RI > 1.42). This sensor provides a label-free, high-performance tool for early warning of postoperative micrometastasis and improves the efficiency of cancer management.