Abstract: In recent years, extreme weather events and geological disasters have frequently triggered overtopping failures of landslide dams and earth-rock dams. To investigate the mechanism of water-soil interaction during the overtopping process, erosion experiments on dam materials are essential. Given the wide gradation of materials in landslide dams and earth-rock dams, conducting erosion experiments on coarse-grained soils and accurately obtaining their erosion parameters remains a key challenge. For an improved Erosion Function Apparatus (EFA) with fixed pipe cross-sectional dimensions, this study employs a three-dimensional pipe model and fluid simulation using Ansys Fluent to determine the optimal geometric shape of the sample cross-section and the maximum allowable particle size. By testing two sample cross-section shapes (rectangular and circular) and four sample extrusion heights, the study examines the impact of different sample parameters on the flow pattern in the erosion section. Results indicate that as the extrusion height increases, the low-velocity fluid region above the sample expands, and flow velocity distribution becomes increasingly uneven. However, compared to rectangular cross-section samples, circular cross-section samples exhibit less disturbance to the fluid above, enabling larger extrusion heights to meet the shear stress measurement requirements of the improved EFA. This enhancement increases the maximum allowable particle size for experiments under fixed pipe cross-sectional dimensions, meeting the measurement needs of erosion characteristics for coarse-grained materials.