In this paper we describe and clarify the mechanisms of water retention dynamics in intertidal sandy flats and beaches. This was done by performing field observations, laboratory experiments and theoretical analyses, which developed in an interactive manner. The combined results demonstrate that the difference in the water retention dynamics for sandy flats and beaches arises from the difference in pore water migrations that take place around air entry suctions for both types of the soils. Furthermore, it is found that for a given sandy soil, there exists a critical ground water level above which the degree of saturation does not decline. Two quantitative index are proposed for assessing and creating such water retention fields. The results obtained can directly apply to the design and management of artificial sandy flats.