Abstract: Artificially designed micro-nano optical devices, such as nanofilms and metasurfaces, are undergoing a revolutionary shift from single-function to multi-function integration, and from passive devices to active ones. Active micro-nano optical devices based on mechanical, electrical, thermal, and other control mechanisms have made impressive progress in dynamically controlling functionalities such as spectral filtering, optical focusing, multidimensional holographic display. However, to enable the broader adoption of active optical devices in real-world scenarios, significant challenges remain in terms of response speed, dynamic range, control area, and other performance aspects. Hydrogel, as a humidity-responsive deformation material, can be fabricated into micro-nano structures that continuously tune their optical response through the nanoscale volumetric expansion driven by ambient humidity variation. The hydrogel micro-nanostructures offer the advantages of rapid response speed, facile fabrication, and wide tuning range, providing an innovative solution for the active control of micro-nano optical devices. This paper starts from the micro-nano fabrication technology of hydrogel materials, proceeding to a systematic review of the dynamic optical tuning capabilities of hydrogel micro-nanostructures and their functional applications such as dynamic filtering, dynamic multifunctional optical display, dynamic fluorescence emission. Finally, we summarize and discuss the full text content and analyze the challenges and future directions of hydrogel micro-nano optical devices.