We present a fast and efficient strategy for the preparation of photonic hydrogels for compression and organic solvent sensing by the self-assembly of monodisperse carbon-encapsulated Fe3O4 nanopartides (NPs). The hydrogel film was composed of acrylamide (AM) and cross-linker N,N'-methylenebis(acrylamide) (BIS), and the formed 1D NPs chain structure can be fixed within the hydrogels under a magnetic field by in situ photopolymerization. The resulting photonic hydrogels display vivid structural color which can be tuned by pressing and organic solvent treatment. The 0.2 kPa compression applied to the photonic hydrogels can be detected by the 37 nm blue shift of a reflection peak. Importantly, the photonic hydrogels can recover to their original state (<1 s) after being compressed on a pattern. Moreover, the sensitivity of mechanochromic photonic hydrogels can be adjusted by manipulating the concentration of monomers, and a large reflection peak shift (4.3 kPa, 200 nm) was observed. The detection range of the compression sensor can thus increase from 0-4.3 to 0-130.6 kPa. The photonic hydrogels are nearly monochromatic, with high sensitivity and stability and fast reversibility, and are potentially useful in displays, diagnostics, compression and solvent sensing.