In situ injectable double-crosslinked hydrogels containing thiol functionalized poly(amido-amine) dendrimers (Gn-PAMAM-NH-X) and oxidized dextrans (ODex) were prepared under physiological conditions without using potentially cytotoxic cross-linkers. The double-crosslinked structure was created by Schiff's base reaction and the formation of disulfide bonds. The morphology of the hydrogels was characterized by scanning electron microscopy. The gelation time, swelling and rheological behaviors of the hydrogels were investigated. We also studied the adhesive strength and cytocompatibility of the hydrogels. The surface amino density, concentration and generation of PAMAM are the main factors affecting the gelation. Relatively high surface amino density contributes to quick gelation, whereas too great a surface amino may lead to the brittleness of the hydrogel. A moderate concentration of PAMAM (10% wt) is suitable for gelation considering its appropriate gelation time. Where surface amino density and the mass concentration of PAMAM-NH were identical, PAMAM with less generation was prone to gelation. The injectable PAMAM/ODex hydrogels have double-crosslinked structures and a high crosslinking density which lead to their high storage modulus. The adhesive strength of the hydrogels is about 2.4 times of commercial available fibrin glue and these hydrogels are nontoxic to L929 mouse fibroblast cells. The L929 cells can attach easily to the surface of hydrogels and proliferate on them, which demonstrates these novel injectable hydrogels are biocompatible and have potential uses in tissue engineering.