Surface plasmon resonance (SPR) biosensors are widely used in sensitive chemical, biological and environmental sensing. Recently, the studies of nano-plasmonics in metallic structures have shown that surface plasmons can also be excited by the metallic nanostructures films which can be used for high-throughput and chip-based SPR type sensing. We developed a class of plasmonic crystal-like structures consisting of a film with arrays of periodic nanoslit geometry. Because the engineered array ensures multiple resonance modes, we use the multispectral analysis to evaluate the refractive index sensing capability. Different from the common method monitoring a single peak shift, the multispectral analysis, observing all the peak shifts and intensity changes in the multiple plasmonic resonances in the spectra, can improve the signal-to-noise ratio of the system and enhance the sensing capabilities. In this investigation, we studied the best condition for the gold nanoslit arrays by testing their ability for refractive index sensing, and a high sensitivity of up to 28586 %T nm/RIU was obtained by multispectral analysis (RIU = refreactive index unit, and T= transmission).