Based on a biodegradable cross-linker, acryloyloxyethylaminopolysuccinimide (AEA-PSI), a series of looser cross-linked poly(N-isopropylacrylamide-co-acrylic acid) [P(NIPAAm-co-AAc)] hydrogels were prepared, and their water content, swelling/deswelling kinetics, and the morphology of the gels were investigated. The swelling behaviors of AEA-PSI-cross-linked P(NIPAAm/AAc) hydrogels were investigated in Dulbecco’s phosphate-buffered saline (pH = 7.4), in the distilled water, and in the simulated gastric fluids (pH = 1.2), respectively. The water contents of the hydrogels were controlled by the monomer molar ratio of NIPAAm/AAc, swelling media, and the temperature. In the swelling kinetics, all the dried hydrogels exhibited fast swelling behavior, and the swelling ratios were influenced significantly by the amounts of AEA-PSI and AAc content. The deswelling kinetics of the hydrogel were independent of the content of AAc and cross-linker. Lastly, the morphology of the hydrogels was estimated by the field scan electron microscopy.