With the development of deformation structured color materials and sensor signal analysis, the combination of photonic crystals with optical structure characteristics and highly sensitive hydrogels in the environment can stimulate response under conditions of mechanical deformation, temperature, humidity, pH, etc. Optical-based hydrogel sensors avoid these disadvantages of conventional sensors, such as non-destructive measurements, high-speed transmission, almost interference-free, telemetry remote control, and many other advantages. This paper introduces the optical properties of photonic crystals and the synthesis of hydrogel polymers, and describes the working principle of photonic crystal hydrogel sensors. There is no systematic generalization of the direction of flexible sensors, so this paper summarizes the progress in the direction of physical, chemical, and biological sensors, and reviews the related research applications, and discusses the potential future directions and development challenges.