To explore the influence of offshore tide on the mechanical response of the retaining structure of foundation pit， the dynamic water level boundary input method is adopted to analyze the attenuation law of groundwater seepage under the action of tides at different amplitudes. Based on the measured groundwater level data near the retaining structure， the inversion method for tidal groundwater level amplitude at the numerical model boundary is established， and the mechanical response of the retaining structure of foundation pit under the action of tidal groundwater at different amplitudes are then studied. This study indicates that with the help of the inversion method for tidal groundwater level amplitude based on the attenuation law of groundwater seepage， the boundary groundwater level required for the small-scale regional numerical analysis of the foundation pit can be obtained without considering the geological information of the actual offshore area in detail， which has a good application value in the analysis of the tidal effect on offshore engineering. The direct contact area between the sea-side retaining structure and the strong permeable layer and the drilled grouting pile in the pit-in-pit are more affected by the cosine tidal waves， with the horizontal lateral swing of these two parts showing a ‘sinusoidal-cosine similarly’ change rule with time. However， due to the self-collaborative deformation and the effect of pore pressure loads with different phases in other strata， the lateral time-history curves of the back-sea side retaining structure and the other regions of the seaside retaining structure show an asymmetric ‘irregular groove’ change rule. Under the action of tide， the lateral displacement amplitude of the retaining wall at the back of the sea gradually decreases with the increase of depth， but the lateral displacement amplitude， the bending moment increment distribution and the internal bracing axial force of the retaining wall at the sea side are closely related to the permeability of the stratum. Especially in the structural design of the project affected by large tide， the maximum lateral displacement position and bending moment peak position of the static working condition should be avoided to coincide with the position of the strong permeable layer as far as possible.