A novel injectable in situ cross-linked hydrogel was prepared from alpha,beta-poly(N-hydroxyethyl)-dl-aspartamide (PHEA), which had excellent biocompatibility and biodegradability. PHEA was modified by acryloyl chloride (AC) via the reaction between hydroxyl groups and acryl groups to introduce reactable double bonds into the chain. Two macromers with different degrees of derivatization were prepared. Through NMR and FT-IR characterizations, the structures of the polymers were proposed. Hydrogels were prepared by covalent cross-linking between double bonds with a transition from liquid to gel at body temperature. The effect factors on gelation time, swelling ratio and gel content were investigated in detail. It was found that the grafting ratios of AC, concentrations of macromer and initiator had great influence on the gelation time. And it could be adjusted to meet the requirements of an injectable material.