Scintillation-based X-ray excited optical luminescence (XEOL) imaging shows great potential applications in the fields of industrial security inspection and medical diagnosis. It is still a great challenge to achieve scintillators simultaneously with low toxicity, high stability, strong XEOL intensity, and weak afterglow as well as simple device processibility with weak light scattering. Herein, we introduce ethylenediaminetetraacetate (EDTA)-capped NaGdF4:10Ce/18Tb nanoparticles (NPs) as a highly sensitive nanoscintillator, which meets all of the abovementioned challenges. These NPs show comparable XEOL intensity to the commercial CsI (Tl) single crystal in the green region. We propose a mechanism that involves a new electron-captured path by Ce3+ ions and the promotion of energy migration from a trap center to surface quenchers via a Gd3+ sublattice, which greatly reduces the population in traps to produce significant reduction of afterglow. Moreover, by employing an ultrathin transparent NaGdF4:10Ce/18Tb film (0.045 mm) as a nanoscintillator screen for XEOL imaging, a high spatial resolution of 18.6 lp mm-1 is realized owing to the greatly limited optical scattering, which is superior to the commercial CsI (TI) scintillator and most reported lead halide perovskites. We demonstrate that doping Ce3+ ions can greatly limit X-ray-activated afterglow, enabling to use an ultrathin transparent fluoride NP-based nanoscintillator screen for high-quality XEOL imaging of various objects such as an electronics chip and biological tissue.