Abstract: The SA/P(NIPAM-co-AM) hydrogel was prepared by in situ free radical copolymerization with different contents of N-isopropyl acrylamide (NIPAM) and acrylamide (AM) in sodium alginate (SA) network. By changing the composition ratio, a series of thermalsensitive hydrogels with different lower critical solution temperatures (LCST) were prepared. And under the condition of a low AM/NIPAM mass ratio, the LCST of the gel is linearly correlated with the increase of the ratio. Meanwhile, the presence of carboxyl group and amino group also makes the gel responsive to pH value, which can expand the application of the hydrogels. Solid-state NMR was used to study the molecular dynamics at different temperatures, especially near the LCST. When the temperature is lower than the LCST, the mobility of molecular chain is quick; when temperature is higher than LCST, the molecular mobility decreases due to the collapse of the network. The mobility of different groups in different frequencies do not change synchronizedly, but in certain sequence. The high-frequency motion of the main chain is first affected by temperature (298 K), followed by high-frequency motion of the side chain (301 K), and finally the low-frequency motion of the side chain (304 K). The low-frequency motion of the main chain is not significantly affected by temperature. The macroscopic changes occur after the reduction of the mobility of all the groups (305 K). In addition, using the low field nuclear magnetic resonance (LF-NMR) the swelling process of composite gel is studied in situ. The diffusion of water molecules and the relaxation of macromolecular chains are synchronized, which conform to the swelling model of non-Fickian diffusion.