A uniaxially oriented poly(vinyl alcohol) (PVA) film was found for the first time to respond to the humidity change in the two different modes under the restrained condition, which are essentially the same as the modes of the supercontraction and cyclic contraction observed in spider dragline silk. Once the atmospheric humidity started to increase, the PVA film showed at first the irreversible stress generation (supercontraction stress), followed by the reversible stress generation synchronizing with the cyclic change of humidity. These irreversible and reversible stress changes have been connected to the changes of higher-order structure caused by the cyclic change of wet/dry atmosphere as revealed by the detailed in situ measurements of the 2-dimensional wide-angle and small-angle X-ray scatterings during these processes. On the basis of a simple mechanical model and the thus-collected information on the higher-order structural change, it was concluded that the irreversible and reversible stress generations are governed mainly by the irreversible hydration-induced stress relaxation in the highly tensioned amorphous region and the reversible swelling in the normal amorphous region, respectively.