An immunosensor based on graphene oxide modified cladding etched long-period fiber grating sensor for detection of avian influenza virus is presented in this paper. Graphene oxide is coated on the surface of long-period fiber grating by hydrogen bonding using sodium hydroxide and the avian influenza virus monoclonal antibodies are combined with graphene oxide surface by a covalent bond. The resonant wavelength change of long-period fiber grating caused by the specific binding of avian influenza virus monoclonal antibody aolsorbed on graphene oxide to avian influenza virus antigen was used for detection. The experimental results show that the limit of detection of the graphene oxide modified cladding etched long-period fiber grating immunesensor can reach 40 ng/mL, the dissociation constant ~1.6×10^-7 M and the detection range 40 ng/mL~200μg/mL. The detection results of avian influenza virus in avian influenza virus blank allantoic fluid, Avian influenza virus allantoic fluid and Newcastle disease virus allantoic fluid show that the immunosensor has good specificity and clinical characteristics. Therefore, the immunosensor could be potentially applied in the fast detection and early diagnosis of avian influenza virus.