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Metal‐free perovskites (MFPs) with flexible and degradable properties have been adopted in flexible X‐ray detection. For now, figuring out the key factors between structure and device performance are critical to guide the design of MFPs. Herein, MPAZE‐NH4I3 ⋅ H2O was first designed and synthesized with improved structural stability and device performance. Through theoretical calculations, the introducing methyl group benefits modulating tolerance factor, increases dipole moment and strengthens hydrogen bonds. Meanwhile, H2O increases the hydrogen bond formation sites and synergistically realizes the band nature modulation, ionic migration inhibition and structural stiffness optimization. Spectra analysis also proves that the improved electron‐phonon coupling and carrier recombination lifetime contribute to enhanced performance. Finally, a flexible and degradable X‐ray detector was fabricated with the highest sensitivity of 740.8 μC Gyair−1 cm−2 and low detection limit (0.14 nGyair s−1).

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Metal halide semiconductors have huge potential on X‐ray detection, due to their high atomic number, adjustable photoluminescence wavelength, superior stability, and simple fabrication process. Here, cesium lead halide nanocrystals (CsPbX3 NCs) are fabricated on paper substrate as scintillator screen for the first time. By means of all‐solution processed spray‐coating method with optimization of polymethyl methacrylate, large‐area (>600 cm2) and low‐cost (0.0975 dollar cm−2) flexible scintillator screens are obtained with high resolution (≈5 lp mm−1), low detection limit (2.97 μGy s−1), high light yield (≈30000 photons/MeV), and fast response speed (<15 ms). The screen can achieve high‐resolution X‐ray non‐distortion imaging for non‐planar objects and realize battery check and safety monitoring at a broad range of photon energy (10–160 keV). In addition, real‐time dose rates can be easily estimated through counting the luminous arcs of the designed visible indicator. More important, the screen shows remarkable tolerance to high‐dose X‐ray radiation (tolerance over 60 Gyair continuous radiation with only 3% performance degradation), water soaking (2000 h), and conventional storage (over 300 days without apparent deterioration under ambient conditions). Hence, this work may provide huge potential in the field of medical diagnosis, non‐planar detection, and safety monitoring in the future.