Abstract: The diameters of suspended loads and bed materials in the Lower Yellow River (LYR) have undergone significant changes since the operation of the Xiaolangdi Reservoir (XLD). To better understand these variations, this study examines the temporal and spatial changes in the diameters of both suspended loads and bed materials. Based on data from hydrologic stations and range surveys, and considering the delayed response of bed material adjustment to variations in flow and sediment input, the evolution of particle diameters is analyzed. A method for calculating the median diameter of bed material is subsequently proposed. The results indicate that during the phase when the XLD released a density current, the suspended load was fine, with a median diameter of 0.0075 mm. In contrast, during the retrogressive scouring phase associated with lowering the reservoir pool level, the suspended load was coarser, with a median diameter of 0.0273 mm. The spatial and seasonal distribution of bed material grain size is characterized by the pattern of “coarse in upstream reaches and fine in downstream reaches, ” as well as “coarse in non-flood seasons and fine in flood seasons.” In the reaches upstream of Jiahetan, the grain size of bed material is mainly influenced by the incoming sediment coefficient and sediment load. Downstream of Jiahetan, it is primarily affected by the incoming sediment coefficient and water discharge. Formulas for computing the median bed sediment particle size in various reaches of the LYR are proposed. The coefficient of determination between the fitted and the observed values exceeds 0.62, with relative errors ranging from 8.68% to 11.91%. Therefore, these formulas can be effectively applied for calculating and predicting bed material size in the LYR. The results provide technical support for flood control operations at the XLD and for improving sediment transport efficiency in the LYR.