Inspired by natural plants, thermoresponding hydrogel (TRH) structures have been designed to trigger mechanical instability with fast actuation. Tough Ca-alginate/poly(N-isopropylacrylamide) (PNIPAM) hydrogel has been synthesized by the hybrid of physically cross-linked alginate and covalently cross-linked PNIPAM. The tough Ca-alginate/PNIPAM hydrogel exhibits 30 kPa of elastic modulus, 280 J/m2 of fracture energies, and fivefold of uniaxial stretch. A multilayered structure made of (Ca-alginate/PNIPAM)/(Ca-alginate/poly (acrylamide)) hydrogels demonstrate fast actuation induced by mechanical instability. A finite-element simulation model is developed to investigate the deformation and to guide the structural design of the hydrogels. The instability-triggering mechanism can enhance the actuation performances of hydrogel structures in applications, such as drug delivery, microfluid control system, and soft biomimetic robotics.