We demonstrate a fiber refractive index (RI) sensor based on an excessively tilted fiber grating (ExTFG) immobilized by large-size plasmonic gold nanoshells (GNSs). The GNSs are covalently linked on ExTFG surface. Experimental results demonstrate that both the intensity of the transverse magnetic (TM) and transverse electric (TE) modes of ExTFG are significantly modulated by the localized surface plasmon resonance (LSPR) of GNSs due to the wide-range absorption band. The wavelength RI sensitivities of the TM and TE modes in the low RI range of 1.333–1.379 are improved by ～25% and ～14% after GNSs immobilization, respectively, and the intensity RI sensitivities are ～599%/RIU and ～486%/RIU, respectively.