Abstract: The paper analyzes the dynamics of groundwater level change in coastal aquifers influenced by tides in the Beihai area based on short- and long-series observations of tidal effects. The results indicate that the groundwater level of coastal aquifers under tidal influence has similar fluctuation features to those of tides but the magnitude of groundwater level fluctuation is smaller than that of tides. The tidal influence on the groundwater level fluctuation or tidal coefficient is related to the distance of the observation station from the coast. The magnitude of groundwater level fluctuation or tidal coefficient roughly decreases exponentially. The groundwater level is thought to be free from the tidal influence when the distances from the southern and northern coasts are 3756 and 2266 m respectively. The data of the long-series observations also show that the tides and groundwater level at the coast fluctuate in a long period of about 15 days and a short period of about 1 day.