The evolution of optical absorption and photoluminescence (PL) spectra with different Ag content (from 0.14 wt % to 2.80 wt %) has been investigated for Ag/mesoporous SiO2 when heating in H2 and air. It has been found that, in accordance with alternating heat treatment of the sample with fairly low Ag content in H2 and air, the surface plasmon resonance (SPR) absorption of Ag nanoparticles alternately and reversibly appears and disappears, whereas its PL emission alternately disappears and appears. However, such reversible change can not happen in the sample with high Ag content. With gradual increase of Ag content, the two kinds of spectra take great and regular change. The results have firstly been discussed based on two contrary mechanisms coexisting in air at high temperature (i.e., oxidation reaction and thermal reduction reaction). In particular, the abnormal oxidation and the great optical change with Ag content is closely relative to surface/interface synergetic effect between Ag, O2 and SiO2 matrix despite SiO2 usually being regarded as a chemical inert medium to noble metals. The study would provide valuable reference for optical switch, gas sensor, nano optical modulation of SiO2-based optical devices, and give valuable information for surface/interface chemistry and catalysis in Ag nanoparticles and SiO2 assemble system.