Mixed crystal strategy is an effective approach of improving the luminescence properties of optical materials and has been adopted widely in many systems. In this paper, the La-mixed Gd2Si2O7:Ce polycrystalline samples were successfully synthesized by a sol-gel method. The crystal structure and luminescence properties were confirmed and discussed by XRD, UV-Vis luminescence spectra, and XEL, respectively. The vacuum ultraviolet excitation spectra and thermoluminescence glow curves were also systematically investigated and discussed at varied temperature. A combination of the first-principles calculations and optical characterization experiments was employed to study the electronic band structure of host material, revealing that the band gap is narrowed and the 5d1 level of Ce3+ shifts to higher energy as the La content increases. The luminescence thermo-stability and activation energy were also measured and calculated. It indicates that thermo-stability is strongly dependent on the La concentration. An effective approach is developed to tune the electronic band structure, luminescence properties and thermostability of (Gd1-xLax)2Si2O7:Ce scintillator by adjusting La/Gd ratio.