One fast and one slow relaxation mode were observed in semidilute aqueous solution of poly(N-isopropylacrylamide) (PNIPAM) by dynamic light scattering (DLS). The length scale of the slow relaxation was found to be comparable with the observation length (1/q) in our DLS measurements. To elucidate its origin, we varied the solution temperature and used different salts to study their effects on such a slow mode because it is known that they can influence the strength of the water-amide hydrogen bonding as well as the icelike structure of liquid water. As the temperature increases, the slow mode was gradually suppressed. Using a strong salting-out salt (Na2SO4) and a strong salting-in salt (NaSCN) with the same cation as two model systems, we found that the salting-out salt suppresses the slow dynamics, while the salting- in salt amplifies it. Our results reveal that the slow mode is related to a long-range correlated density or concentration fluctuation, namely, the interaction among the segments in different blobs on chains.