In order to improve the sensitivity and limit of detection of 81° tilted fiber grating, bovine serum albumin immunosensor based on graphene oxide modified cladding-etched 81° tilted fiber grating was proposed, the principle and sensing properties of the sensor were analyzed. Refractive index sensitivity of 81° tilted fiber grating was improved by etching with hydrofluoric acid. Then the etched 81° tilted fiber grating was coated with graphene oxide, and bovine serum albumin monoclonal antibody were modified on the surface of 81° tilted fiber grating for the specific detection of bovine serum albumin. Experimental results show that the detection range of the immunosensor is 0.15~15 nmol/L, limit of detection ~0.165 nmol/L, and the sensitivity in its linear response region is ~182 pm/(nmol·L^-1). Although the detection range decrease, the sensitivity is ~5.3 times of that of the un-etched graphene oxide coated 81° tilted fiber grating, and the limit of detection is also improved.