The large range of sea surface of Sanggou Bay is covered by kelp breeding shelving and the original local hydrodynamic vertical structure. According to the analysis of the measured data from winter 2023, it is found that the surface flow rate at the observation point during the winter-spring tide is approximately 2 cm/s lower than that of the subsur-face flow rate. To explore the influence of kelp farming on the hydrodynamics of Sanggou Bay, In this paper, we studied the effect of kelp on hydrodynamic force using the MIKE3 model; four conditions of different growth periods of kelp were set, The effect of kelp growth on the hydrodynamics of the Sanggou Bay, research finding, Breeding rack in Shanghai belt growth will reduce the flow rate by about 30%, The longer the kelp length is, The more is the weakening of the hydrodynamic, By analyzing the elliptic characteris-tics under four conditions, It was found that large areas of kelp growth in Sanggou Bay affected O1, K1, M2 and S2 tidal points, The long axis reduced 8.3cm/s, 10.9cm/s, 3.2cm/s and 1.1 cm/s, respectively, And under the influence of the vertical and horizontal lines of seedlings, The direction of the O1 and K1 minute tide is also affected to some extent. The model calculated the change in the semi-exchange period in the Sanggou Bay area after kelp growth and found that kelp growth altered the semi-exchange period in Sanggou Bay from 7.5 days to 10.5 days, thereby reducing its water diffusion ability.