Abstract: The coastal sedimentary environment serves as the fundamental basis for comprehending the evolution and movement of groundwater. To gain a comprehensive understanding of the distribution and mobility of brackish groundwater in the southern coast of Laizhou Bay, two archetypal Quaternary boreholes (LZ01 and LZ02) were sampled, and porewater was extracted through mechanical squeezing, and the grain size composition of sediments and the Cl- content of porewater were determined. Both boreholes are dominated by fine-grained sediments with an average particle size of 5.5φ.By examining the characteristics of sediment lithology and grain size parameters, it was discovered that LZ01 and LZ02 boreholes contained 10 and 9 alternating sedimentary facies, respectively, indicating three sets of marine stratigraphic units since the late Pleistocene period. The porewater was found to be predominantly composed of brine (saline) water, with the Cl- profile displaying three peaks corresponding to the three sets of marine stratigraphic units. These results demonstrate the presence of residual marine ancient brine water in the sediment and its permeation into neighboring sediments. In the study, it was observed that the hydraulic index of sedimentary water exhibited a clear zonal pattern, with the vertical change rate of Cl- serving as a crucial indicator. Notably, under moderate hydraulic conditions, Cl- displayed a robust diffusion and migration capability in pore water, as confirmed by sensitive particle size analysis. The dominant particle size range for pore water migration was found to be between 45.61-111.47μm. Additionally, the distribution of brine was controlled by ancient marine invasion events, while low permeability and lithology played a crucial role in sealing the ancient brine. Overall, these research findings provide a strong theoretical foundation for comprehending the genesis, salinization, and transportation of brine in the Laizhou Bay coastal plain.