Abstract: 11 columnar sediment samples are collected off the mouth of Dingzi Bay for grain size analysis and ²¹⁰Pb radioactivity measurement. With the results, the sediments are classified and grain size parameters and sedimentation rate calculated. Combined with the geomorphologic evolution of Dingzi Bay and the change in human activities in the Wulong River Basin, the influence of human activities on the evolution of sediment grain size in the coastal section out the Wulong River estuary is discussed. The results suggested that the columnar sediments may be divided into seven types, and the mean grain size (Φ) in the horizontal direction shows a trend of increase from the deltaic center to all directions as well as an irregular pattern in vertical direction. The sediment sorting changes from good to medium, with a near-symmetric-positive skewness and sharp kurtosis. Sediment types and their diagnostic parameters may be used as tools to reveal depositional environments. Sand, gravelly and silty sand and their small Φ-values suggest relatively strong hydrodynamic environments, while sandy silt, silt, clay and mud and their large Φ-values indicate relatively weak hydrodynamic environments. The average sedimentation rate at the top and front of the delta was 0.51 cm·a⁻¹ and 1.19 cm·a⁻¹, respectively. Human activities in beach reclamation and others increased since 1980, which have caused the decrease in tidal space, sediment discharge and velocity into the sea. As the results, the overall trend of sediment particles in the human-influenced area becomes finer.