Abstract: Column experiments with sediments in the Beidagang reservoir for the coastal region of Tianjin were conducted to elucidate the dynamic characteristics of colloid release and salt release or retention of sediments in different positions in the exchange process of saline groundwater and reservoir water, and to discuss the mechanism of colloid release, salt release or retention for different sediments. Results indicate that the fates of salinity in different locations in the reservoir were different in the exchange process of saline groundwater and reservoir water. Salinity was intercepted by the sediments at the entrance to the reservoir, and released from the sediments and transferred to the water at the reservoir outlet. The cumulative release capacity of colloids from the sediments increased gradually with increasing pore volumes, and the accumulation adsorption curve of colloids conformed to a straight line by the sediments with pore volume; the release rate of the colloids was stable. The maximum cumulative release capacity of colloids from the sediments of the entrance, center and outlet of the reservoir were 3.275 mg/g, 0.386 mg/g and 1.382 mg/g respectively. The release or retention of salt from the sediments in the reservoir was related to many factors, such as particle size, colloid content and salt content. The water salinity and sediments in the reservoir were in dynamic equilibrium. When the salinity of the sediments was higher than that of the equilibrium concentration, the salt from the sediments was released into the water. The salt adsorbed by the colloid was released into the water with the release of the colloid. Contrarily, the salt in the water was intercepted by the sediments. The smaller the particle size of the sediment, the easier the salt in the water was adsorbed. The law of colloid release can be explained using this double layer theory.