Double network (DN) hydrogels have drawn much attention due to their high mechanical strength and toughness with high-water content. In this investigation, we successfully prepared extremely stretchable, transparent and tough DN hydrogels by using neutral synthetic polymer–poly(vinyl alcohol) (PVA) as the first network, polyacrylamide (PAM) as the second network and sodium alginate (SA) as a molecular stent. It was found that the content of SA significantly influenced the swelling property of PVA semi-interpenetrating polymer network (semi-IPN) hydrogel in acrylamide aqueous media and corresponding mechanical properties of DN hydrogels. The hydrogels with nearly 90 wt% water had achieved fracture strain of about 2400% and a maximum compression stress of 35.2 MPa. Moreover, we also observed the hydrogels could quickly recover to their initial shapes after the deformation force was removed. By introducing a molecular stent, we anticipated that a large number of neutral synthetic polymers would be utilized to form the first network inDN hydrogels, significantly expanding the scope of the DN hydrogel concept.