Bays represent highly dynamic and complex marine ecosystems, where variations in surface sediment grain size serve as critical indicators of hydrodynamic conditions and sediment provenance. With the intensification of human activities along coastlines, anthropogenic impacts on bay environments have increasingly surpassed the influence of natural processes. This study aims to elucidate the key controlling factors of surface sediment grain size distribution and its temporal and spatial variability. Hydrological data and surface sediment samples were collected from 40 stations across Weihai Bay. By employing comparative analyses of surface sediment grain size distribution and sediment transport models, this research systematically examines the sedimentary environment of Weihai Bay. Results indicate that the spatial distribution of sediment grain size is predominantly governed by tidal currents. In 2023, the surface sediments of Weihai Bay exhibited a generally coarser grain size compared to 1988, driven primarily by changes in tidal current strength and sediment supply. Human activities, including the alteration of tidal current patterns and modifications to sediment sources, have been identified as the main contributors to the observed spatiotemporal shifts in sediment grain size. Furthermore, with the continued expansion of anthropogenic activities, such as land reclamation, river channelization, and coastal infrastructure development, the coarsening trend of surface sediments in Weihai Bay is expected to persist. This research provides critical insights into the mechanisms through which human activities influence sediment grain size dynamics in coastal environments and offers a scientific foundation for coastal zone management and sustainable development strategies.

surface sediment grain size characteristics, grain size coarsening, human activities