A biodegradable polymer network hydrogel system with both hydrophilic and hydrophobic components was synthesized and characterized. The hydrophilic and hydrophobic components were dextran and poly(D,L)lactic acid (PDLLA), respectively. These two polymers were chemically modified for incorporating unsaturated groups for subsequent UV crosslinking to generate a hydrogel with a three-dimensional network structure. The effects of the reaction conditions on the synthesis of a dextran derivative of allyl isocyanate (dex-AI) were studied. All newly synthesized materials were characterized by Fourier transform infrared and NMR. The swelling property of the hydrogels was studied in buffer solutions of different pHs. The results of this study showed that a wide-range swelling property was obtained by changes in the dex-AI/PDLLA composition ratio, the type and degree of unsaturated groups incorporated into dextran, and the UV photocrosslinking time. The solvent extraction effect on the swelling property of the hydrogels was also studied.