This study proposed a graphene oxide （GO）-integrated excessively tilted fiber grating （ExTFG） local surface plasmon resonance （LSPR） immunosensor for the label-free and specific detection of the cancer biomarker programmed cell death-ligand 1 （PD-L1）. First， large-sized （165 nm） gold nanoshells were used as LSPR carriers to be immobilized on the fiber surface； then， GO was coated on the fiber surface to improve the hydrophobic properties of the ExTFG-LSPR sensor to enhance the sensor's ability to adsorb biomolecular proteins. Finally， anti-PD-L1 monoclonal antibodies were covalently bound to the carboxyl terminal of the GO on the fiber surface for the specific detection of PD-L1. Experimental results showed that the detection range of the immunosensor for PD-L1 antigens in phosphate buffer is 0.038–38.46 pmol/L， exhibiting suitable linearity in the low concentration region of 0–2 pmol/L with a sensitivity of approximately 0.114 nm/（pmol·L^-1）. The detection limit of the immunosensor for the PD-L1 antigen dissolved in phosphate buffer is as low as approximately 0.076 pmol/L， and the dissociation coefficient is approximately 2.801×10^-12 mol/L. When the immunosensor was used in clinical immunoassays with different healthy serum samples and hepatocellular carcinoma serum samples， its responses to the former were extremely weak but were noticeable toward the latter， indicating that it had suitable clinical specificity for PD-L1 in complex serum environments and thus showed potential for clinical application.