Using a thin film double-network (DN) hydrogels of 100 mu m thickness, the behaviors of DN gels, from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) as the first network and polyacrylamide (PAAm) as the second network, swelled in poly(ethylene glycol) (PEG) aqueous solution of short molecular weight (M-w: 400 g/mol) are investigated. Comparing with the conventional DN gels that are swelled in water, the toughness of the DN gels swelled in PEG solution is largely enhanced, as revealed by the tensile test and tearing test. Furthermore, a transition in the crack propagation dynamics, from the steady-state mode in water to the stick-slip instability mode in PEG solution, is observed in the tearing test. The morphology around the crack front of the DN gels also changes with the presence of PEG, and characteristic triangular-like damage patterns are observed. Besides, spicule-like structures are observed on the fractured surface, either in water or in PEG solution. The PEG-induced behaviors are associated with the molecular interaction of PEG to polyacrylamide, the second ductile network of the DN gels.