This work investigated the potential of the silicon-doped magnesium oxide nanotubes (Si: MgONT) to serve as a photocatalyst for the treatment of pollutants. The analysis of the photocatalytic properties of the Si: MgONT was carried out based on considering structural, electronic and optical properties at Si concentrations of 3.12% and 6.25%, respectively. We performed ground state analysis and ionic interactions using density functional theory (DFT) via quantum ESPRESSO and Yambo codes. The results of structural property analysis showed that pristine single-walled magnesium oxide nanotubes (SWMgONT) were stable to the introduction of Si impurities at a concentration of up to 6.25%. The highest binding energy value of −288.66 eV for 3.12% Si-doped SWMgONT showed that photons can be bound more strongly in this system than for 6.25% Si-doped and pure SWMgONT. 3.12% Si-doped SWMgONT exhibited indirect band gaps of 2.36 eV, which is well above the standard overpotential for pollutant degradation, while 6.25% SWMgONT had no bandgap. Analysis of the optical absorption spectra showed that 3.12% SWMgONT absorbs light very well in the visible region and reflects it in the IR region, while pristine and 6.25% MgONT showed poor light absorption in the visible region. On this basis, this work recommended 3.12% Si-doped SWMgONT semiconductor as a better material for dye degradation, CO2 reduction and hydrogen evolution.