Developing scintillators with high light yield (LY), superior irradiation stability, and weak afterglow is of significance for the realization of low-dose high-resolution X-ray excited optical luminescence (XEOL) imaging. Lanthanide doped fluoride nanoparticles possess low toxicity, superior environmental stability, facial fabrication process, and tunable emissions, which are appropriate candidates for the next generation nanoscintillators (NSs). However, the low LY and strong positive hysteresis greatly restrict their practical application. Here, we propose an effective strategy that engineers energy gap to significantly enhance the LY. Our results verify that the tetragonal LiLuF₄ host benefits the crystal level splitting of Tb³⁺ ions, which greatly promotes the electrons population on the Tb³⁺:⁵D₄ level followed by the enhanced LY. The LY of LiLuF₄:Tb@LiLuF₄ NSs is calculated to be ~ 31,169 photons/MeV, which is much higher than the lead halide perovskite colloidal CsPbBr₃ (~ 21,000 photons/MeV) and LuAG:Ce (~ 22,000 photons/MeV) scintillators. Moreover, the positive hysteresis is remarkably restricted after coating a thin shell. The X-ray detection limit and spatial resolution are measured to be ~ 21.27 nGy/s and ~ 7.2 lp/mm, respectively. We further verify that this core/shell NS can be employed as scintillating screen to realize XEOL imaging under the low dose rate of 13.86 μGy/s. Our results provide an effective route to develop high performance NSs, which will promote great opportunities for the development of low-dose high-resolution XEOL imaging devices.