Although supercontraction of animal silks has been extensively investigated, reports regarding their humidity-induced actuation behavior, i.e., the cyclical response to humidity changes, are limited. Humidity-induced actuation is a unique characteristic of animal silks that responds to changes in surrounding humidity for matching the needs of biological functions. Therefore, the aim of this article is to systematically understand the interplay between Antheraea pernyi silks and water from an experimental and theoretical perspective. In particular, the cyclical response of silk to humidity has been primarily investigated. The reversible ordered GGY motifs in the amorphous region of silk are proposed to play a crucial role in humidity-induced actuation relied on the results from synchrotron Fourier transform infrared microspectroscopy as well as small- and wide-angle x-ray scattering. In addition, using the group interaction model, a model based on mean-field theory for understanding the energy storage and dispersion during mechanical loading, the entire humidity-silk response process, including both supercontraction and humidity-induced cyclic actuation, was well quantified. This fundamental understanding of the interplay between silk and water is not only crucial for analyzing the biological function of animal silks, but is also especially critical for inspiring the design of humidity-driven fiber actuators.