A novel injectable in situ cross-linked hydrogel has been designed via Michael type addition between thiol-modified chitosan (CS–NAC) and PEG diacrylate (PEGDA). Hydrogel was rapidly formed in situ under physiological conditions. The gelation time depended on the content of free thiols in CS–NAC, temperature, and concentration of CS–NAC and PEGDA. Thermogravimetric analysis showed the thermal stabilities of hydrogels. SEM observation results confirmed a porous 3D structure. Rheological studies showed that the cross-linking density and elasticity of hydrogel had a correlation to the content of CS–NAC and PEGDA. Swelling studies revealed that these hydrogels had a high initial swelling and were degradable under physiological conditions. And swelling was highly temperature-dependent and was directly related to the amount of cross-linking. Biological activities of the hydrogels were evaluated by in vitro cell compatibility on HDFs and A549 cells and the results indicated that the hydrogel was biocompatible.