<p>Oxidation is one of issues obstructing the applications of Ag nanoparticles (NPs) related to local surface plasmonic resonance (LSPR). In this work, the effects of surface oxidation and passivation on the extinction and near-field enhancement of Ag NPs on glass substrate are studied by a finite-difference-time-domain method. We clarify for the first time that the contact of Ag NPs with SiO₂ substrates results in the appearance of plasmonic mode at the bottom (<i>b</i>-modes), which distinguishes from the LSPR occurring on the full surface (<i>s</i>-mode). With the decrease in contact angle, <i>b</i>-mode is enhanced and becomes predominant in extinction and near-field enhancement, while <i>s</i>-mode contributions are gradually reduced. As Ag NPs are oxidized, <i>b</i>-mode is red-shifted with the increase in Ag₂O thickness and the near-field enhancement will rapidly quench. Besides the influences of the decrease in metallic Ag NP size and the increase in Ag₂O thickness, the LSPR damping is the major reason leading to the quenching of near-field enhancement. If using a layer of dielectric materials to protect Ag NPs against oxidation, SiO₂ or the dielectric material with high refractive index is found of benefit to the applications of enhancing the Raman signals and fluorescence.</p> Graphical Abstract <p></p>