Incorporating flexible cross-links into a brittle network for hydrogel not only significantly improves its toughness, but also effectively provides opportunities to design novel double network (DN) hydrogels with targeted properties and multi-functionalities. However, the principles and toughening mechanisms behind many of experimental studies have not been investigated. In this paper we proposed, for the first time, a phenomenological model for the DN hydrogels, which were derived from homogenous tetra-polyethylene glycol (tetra-PEG) first network, molecular stent and flexible polyacrylamide (PAAm) second network, and theoretically and then experimentally studied the pre-swollen effects on their ultra-stretchable yielding strains. Based on the Flory-Huggins solution theory, the pre-swollen effect of polar molecular stents on the mechanical properties of tetra-PEG/PAAm DN hydrogels was investigated. Finally, the constitutive stress-strain relationships of two-stage loading process were established for the overall response of mechanical behavior, and a good agreement between the numerically simulated results with the experimental ones has been achieved. This study provides a fundamental understanding of the working mechanism of pre-swollen effects and the design guidance for the ultra-stretchable and toughened DN hydrogels.