Spatial resolution is an important criterion to evaluate the performance of a scintillation screen for X-ray imaging. Perovskite-based X-ray screen, usually made of powders or polycrystalline films, suffers from low spatial resolution (~ 200 μm) due to the large thickness of scintillation layer despite of their compelling sensitivity to X-ray dose. In this work, a concentrated colloid of CsPbBr₃ nanosheets was synthesized via a co-precipitation method at ambient condition. By drop casting, smooth scintillation screens of varied thickness were formed through self-assembly, which exhibited both high internal and external photoluminescence quantum yield (PL QY) (84.5% and 75.1%, respectively). The screen-based X-ray detector showed a high sensitivity down to 27 nGy/s, two orders of magnitude lower than the regular dose for medical diagnostics. Importantly, the screen of optimal thickness of 15 μm showcased an unprecedented spatial resolution (26 μm) when used for X-ray radiography, representing one order of magnitude improvement in perovskite community.