In this work, we present a fast and efficient strategy for the preparation of responsive photonic hydrogels for aldehyde sensing by combining the self-assembly of monodisperse carbon-encapsulated Fe3O4 nanoparticles (NPs) and in situ photopolymerization of polyacrylamide (PAM) hydrogels. The responsive photonic hydrogels exhibit structural color variation after being treated with formaldehyde aqueous solution, which can be attributed to the chemical reaction between the amide groups in the hydrogels and the formaldehyde. We have also shown that the photonic hydrogels can be used to determine the concentration of formaldehyde and to differentiate aldehydes through a facile reflection spectra shift and color change. This study provides a facile strategy for the visualized determination of aldehyde in aqueous solution.