Plasmonic metal nanostructures have promising applications in biosensing due to their ability to facilitate light–matter interaction. However, the damping of noble metal leads to a wide full width at half maximum (FWHM) spectrum which restricts sensing capabilities. Herein, we present a novel non-full-metal nanostructure sensor, namely indium tin oxide (ITO)–Au nanodisk arrays consisting of periodic arrays of ITO nanodisk arrays and a continuous gold substrate. A narrow-band spectral feature under normal incidence emerges in the visible region, corresponding to the mode-coupling of surface plasmon modes, which are excited by lattice resonance at metal interfaces with magnetic resonance mode. The FWHM of our proposed nanostructure is barely 14 nm, which is one fifth of that of full-metal nanodisk arrays, and effectively improves the sensing performance. Furthermore, the thickness variation of nanodisks hardly affects the sensing performance of this ITO-based nanostructure, ensuring excellent tolerance during preparation. We fabricate the sensor ship using template transfer and vacuum deposition techniques to achieve large-area and low-cost nanostructure preparation. The sensing performance is used to detect immunoglobulin G (IgG) protein molecules, promoting the widespread application of plasmonic nanostructures in label-free biomedical studies and point-of-care diagnostics. The introduction of dielectric materials effectively reduces FWHM, but sacrifices sensitivity. Therefore, utilizing structural configurations or introducing other materials to generate mode-coupling and hybridization is an effective way to provide local field enhancement and effective regulation.