Study of the soil water dynamic and water balance in coral islands is important to utilize and manage the limited freshwater resources of islands. In this study, we studied the soil water dynamic in Zhaoshu Island, Xisha archipelago using the observed data and the Richards equation, and analyzed the water balance of this island in the dry and wet seasons, respectively. The soil water movement showed that there was a dry soil layer at the depths between 40cm and 160cm of the soil profile in the dry season. The evaporation (Es) flux was low, but the transpiration (Er) could still absorb water from the capillary zone to maintain growth. In the wet season, the infiltration dominated the main process of soil water movement. The result of water balance showed the precipitation, the change values of water in the entire flow domain (△Volume), bottom boundary flux (vBot), Er, Es were 913mm, 10mm, 349mm, 203mm, 351mm, respectively from October 2018 to September 2019. Precipitation is the only source of the freshwater, about 38% of the precipitation infiltrated into the groundwater, 22% of the precipitation was uptaken by the vegetation, and 39% of the precipitation was evaporated from the land surface. In the dry season, the evapotranspiration (ETa) was just 44mm/month which was 94% larger than the precipitation, and about 14mm/month of water was uptaken from groundwater by the plants. But in the wet season, 56% of the precipitation infiltrated into the groundwater, 37% of the precipitation was consumed by ETa. This study can help us well understand the process of water movement in coral islands, and provide references for further management in protecting coral island ecologies.