<p>At present, most leuco-dye-doped dosimeters exhibit detection limits for γ-rays in the range of tens to hundreds of Gy, which remains insufficient for numerous industrial and medical applications requiring lower radiation doses. Herein, a novel highly sensitive photonic hydrogel radiochromic film is reported. This is the first fabrication of methacrylic anhydride modified Gelatin (GelMA) based carbon-coated Fe₃O₄ nanoparticles (Fe₃O₄@C NPs) film with a magnetic field-induced one-dimension (1-D) periodic structure for γ-irradiation detection. Upon exposure to γ-irradiation, the active vinyl-containing GelMA matrix undergoes cross-linking-induced volume contraction, leading to alterations in the periodic structure of Fe₃O₄@C NPs and consequent structural color changes. This photonic film exhibits a high sensitivity of radiochromic behavior, displaying clear green change even at a low dosage of 5&#xa0;Gy γ-ray. This magnetic field-induced construction of the photonic periodic structure is a much simpler method than that of typical opal or inverse opal. In addition, optimized radiochromic behaviors are achieved by choosing appropriate nanoparticles (e.g., sizes or additive amounts).</p> Graphical Abstract <p></p>