By collagen skin (CS) we mean a material with only collagen fibres in a natural animal skin. Following continuous research of collagen biochemistry, CS shows its superiority in various applications such as leather, wound dressing and absorbing materials. This study discovered the water-induced shape memory effect of CS and exploited the corresponding mechanisms in terms of its molecular and morphological structure. It is revealed that CS can completely fix into a temporary shape and retain a high shape recovery ratio (>90%) under repeatable memory cycles. The reversible cleavage and re-formation of interpeptide hydrogen bonds can work as a "switch" to trigger the shape deformation while peptide chains together with cross-links within collagen work as a "netpoint" for shape recovery. Such water-responsive adaptable mechanical behaviour (a switchable modulus) allows CS to be either used as a robotic handle for loading or as a flexible material for covering. This discovery will provide new opportunities and guidance in emerging research areas such as shape memory devices, biomimetic actuators and artificial skins as well as the physiological function of an animal skin.