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The impact of tidal oscillations on groundwater in coastal reclamation land demonstrates the complex hydrodynamic interaction between seawater and coastal hydrological aquifer systems. The tidal action not only affects the temporal variability of groundwater levels but also exerts a significant influence on the groundwater gradients of salinity within the subsurface aquifers. This study takes the Songmu Area as an example to investigate this ocean–groundwater interaction. Songmu Area is located on a peninsula with coastal land reclamation in Dalian, China. Field campaigns were conducted in this area to measure the tidal action and groundwater parameters in a coastal reclaimed area at artificial backfill layers with pressure and salinity sensors, where the tidal signal can influence groundwater levels and salinity up to a one-kilometer range of inland. Tidal changes in the surface of the sea can be broken down into a number of simple, regular harmonic vibrations, each of which is called a tidal split. The tide and groundwater data were extracted using an enhanced harmonic analysis method. The fluctuations of groundwater levels and salinity were decomposed in response to the periodic tidal oscillation. Various constituents of tide attenuation in the coastal groundwater system were investigated. Our research shows that there is an exponential reduction in the fluctuating amplitude of groundwater levels and the groundwater salinity as distance further inland from the coast. The constituents of tide M₂ (the period is 12.42 h of semi-diurnal tides, S₂ (the period is 12.00 h of semi-diurnal tides), K₁ (the period is 23.93 h of full-diurnal tides), and O₁ (the period is 25.82 h of full-diurnal tides) behave differently for the tidal wave propagation and salinity variability in the coastal aquifer of reclaimed land. Among those constituents, M₂ and S₂ exhibit a higher degree of attenuation compared with K₁ and O₁. Understanding the relationship between groundwater levels and tidal fluctuations in coastal backfill areas is crucial for effective groundwater management strategies and mitigating the adverse impacts of seawater intrusion. This study can serve as a good understanding for assessing the impacts of various mitigation strategies.