To clarify the influence of ocean tidal fluctuations on the dewatering engineering of deep foundation pits, an unsteady seepage model for the deep foundation pit with suspended curtain under tidal fluctuations was established. The semi-analytic solution of real domain space is obtained by Laplace transform, Fourier cosine transform, inverse transformation and Stehfest numerical inverse transformation. Subsequently, the semi-analytic solution is degenerated to a known solution, and a numerical model is established based on COMSOL, so as to verify the accuracy of the semi-analytic solution. Then, the influencing parameters of the semi-analytic solution are analyzed, and the results show that tidal fluctuations affect the dewatering effect of the foundation pit, peak tide level reduces the influence of the pumped well, while low tide level intensifies its influence. We suggest that the water curtain insertion ratio of the coastal foundation pit should be no less than 0.6 to alleviate the influence of the tide on the dewatering. With the increase of the dimension ratio, coastal foundation pits need to increase the amount of water pumped to meet the requirement of dewatering. The anisotropy of soil permeability coefficient has a great influence on groundwater seepage and should be considered in the design of actual dewatering engineering.