In recent years, coastal reclamation has become an important way for coastal areas to ease the contradiction between supply and demand along the land and to develop and utilize marine resources. However, large-scale coastal reclamation will change the original natural properties of the sea area and cause changes in the surrounding hydrodynamic environment. Although coastal geomorphic features have a non-negligible impact on tidal energy loss and disaster prevention, few studies have paid attention to the intrinsic connection between the complexity of coastal morphology and the rate of tidal reduction. In this study, Hangzhou Bay is selected as the research object, and a model based on the correlation between coastal geomorphic complexity and tidal energy reduction rate is constructed by using the fractal geometry theory and the quantitative evaluation method of landscape complexity, and the model is used in the assessment of the impacts of reclaimed land in Zhoushan Islands on the geomorphic complexity and tidal energy dissipation. The results of the study show that the differential tide reduction rate is highly correlated with the complexity of islands, shoreline irregularities, curved boundaries and spatial morphology. In the application of the model, it was found that the traditional planning and design of reclamation led to a dramatic change in shoreline morphology, and with the significant reduction of the number of subdimensions D and the shape index S, the differential tidal reduction rate would be reduced by more than 88%, which also poses a significant threat to coastal and downstream estuarine bay disaster prevention. Finally, based on the above analysis, effective control indexes and scientific reclamation measures are proposed to provide theoretical basis for the efficient utilization and protection of mudflats in China.