In this work, the effects of the Mg layer thickness on the structural, morphological, optical and plasmon-dielectric spectral interactions in the Mg/Bi2O3 films are investigated by means of X-ray diffraction, scanning electron microscopy, optical absorbance and reflectance spectroscopies. It was observed that, the polycrystalline Bi2O3 become amorphous and recrystallizes when deposited onto 50 and 100 nm and 200 nm thick Mg layer, respectively. The optical absorption coefficient and dielectric spectroscopic analysis and modeling revealed a remarkable shrinkage in the energy band gap from 2.49 to 2.15 eV associated with a pronounced enhancement in the dielectric properties of the Bi2O3 upon Mg interfacing and thickness controlling. In addition, while the hole scattering time and hole drift mobility increased with increasing Mg layer thickness, the free holes concentration and hole bounded plasmon frequency significantly decreased. Particularly, as the Mg substrate thickness increases from 50 to 100 and reaches 200 nm, the plasmon frequency decreased from 13.9 to 11.1 and reaches 6.54 GHz, respectively. This wide tunability of the surface plasmon resonance frequency, which was achieved via Mg thickness layer control, are promising for the use of the Mg/Bi2O3 layer as plasmonic surface stabilizers and as microcavities.