Abstract: Taking Sanmen Bay in Zhejiang Province as the research area, the hydrodynamic characteristics of Sanmen Bay and its surrounding waters in 2000, 2009, and 2019 were simulated based on the Mike21 hydrodynamic model, and the impact of the reclamation project on the tidal volume and water flow in Sanmen Bay was analyzed and discussed. The calculation result of exchange capacity proves that in 2000, 2009, and 2019, the maximum tidal volume was 5.06×10⁹m³, 4.95×10⁹ m³, and 4.85×10⁹ m³, and the minimum tidal volume was 1.92×10⁹ m³, 1.90×10⁹ m³, and 1.80×10⁹ m³, on average of 3.55×10⁹ m³, 3.39×10⁹ m³, and 3.21×10⁹ m³, respectively. The average tidal volume from 2000 to 2009 decreased by 4.51%, and from 2009 to 2019, decreased by 5.31%. The water exchange capacity was quantitatively analyzed by conservative substance concentration diffusion. The semi-exchange duration in 2000 was 21-27 d, in 2009 was 23-29 d, and in 2019 was 23-30 d. The time for the concentration of conservative substances in the bay reaching 90% in the three stages was 58 days in 2000. In 2009, it extended additional 7 days to 65 days, and continued to increase to 69 days in 2019. The reclamation of Sanmen Bay has weakened the hydrodynamic conditions and water exchange capacity in the bay. The research results provide a scientific basis for the analysis of the impact of the reclamation project on the hydrodynamic environment of Sanmen Bay, and also a basis and reference for the decision-making of the construction and management of Sanmen Bay.