High-resolution X-ray imaging is increasingly required for medical diagnosis and large-area detection. However, the issues of scattering and optical crosstalk are limiting the spatial resolution of the indirect X-ray imaging. In this study, a feasible and efficient strategy is proposed to in situ synthesize flexible Cs 3 Cu 2 I 5 :2%In + @paper as a superior scintillator film, which can be scaled up to an ultra-large area of 4800 cm 2 . The as-obtained Cs 3 Cu 2 I 5 :2%In + @paper performs a fascinating photoluminescence quantum efficiency up to 88.14%, a steady state light yield of 70169 photons/MeV, and spatial resolution of 15 lp mm -1 . Moreover, the suppressed physical scattering and optical crosstalk of the corresponding film are demonstrated. Accordingly, this work explores a feasible fabrication of customizable scintillation films with large area for high-resolution X-ray detection.