In this study, a novel immunosensor based on graphene-oxide (GO)-encapsulated Au-nanoshell (EGO-AuNS)-coated long-period fiber grating (LPFG) is presented for avian influenza virus (AIV) detection. The EGO-AuNS composites were obtained by coating GO on the surface of AuNS via electrostatic bonding, and immobilized on the surface of LPFG via covalent bonding using a silane coupling agent. Then, AIV monoclonal antibodies (AIV-MAbs), which serve as specific biomolecule recognition units, were fixed on the grating surface to realize the EGO-AuNS-LPFG immunosensor. The experimental results indicate that the refractive index (RI) sensitivity of the EGO-AuNS-LPFG immunosensor is -66.60 nm/RIU when RI is from 1.333 to 1.411, which is six times greater than that of the LPFG-based sensor. Based on the detection of AIV antigen solutions with different concentrations, the limit of detection of the immunosensor is observed to be approximately 8 ng/mL. Further, the immunosensor becomes saturated at approximately 50 μg/mL and exhibits a linearity sensitivity of approximately 2946.25 pm/(μg·mL ^-1), which is 7.3 times greater than that of the GO-coated cladding-etched LPFG immunosensor. Based on the detection results of several allantoic fluids, the proposed immunosensor exhibits good specificity and clinical effectiveness. Furthermore, it exhibits considerable promise for application in the biomedical field.