Semi-interpenetrating polymer network (semi-IPN) and fully interpenetrating polymer network (full-IPN) hydrogels composed of sodium alginate (SA) and N-isopropylacrylamide (NIPAAm) were prepared with Ca2+ and N,N′-methylenebisacrylamide (BIS) as the cross-linkers, respectively. The influence of the SA content and crosslinking degree of alginate on thermosensitive, swelling, mechanical, morphological, and thermal properties was investigated in detail. The hydrogels obtained exhibited obvious thermosensitivity and rapid swelling rate. The presence of Ca2+ contributed to the improvement of mechanical properties obviously, without altering the thermosensitivity and network porosity of the hydrogels significantly. The compressive strength of full-IPN hydrogel was improved considerably, while the tensile strength was increased by 308.5% than semi-IPN hydrogel. The Tg of full-IPN dried hydrogel ran up to 142.9 °C with respect to 97.3 °C of pure PNIPAAm, and the improvement indicated that hydrogel with more compact structure was prepared.