A novel Fe3O4@SiO2/PEG acrylate photonic crystal humidity sensor has been fabricated by combining magnetic assembly and photo-polymerization. The sensor self-displays brilliant colors as the humidity changes. By optimizing the cross-linking level and the thickness of the polymer film, the reflection wavelength can shift 160 nm in several minutes, while its intensity is readable by the naked eye. The tested sensors demonstrate good reversibility and durability in cycling experiments. Such photonic crystal sensors may have great potential as energy efficient devices, as no electric power is required for their operation.