The Sanlongsha dune field, on the northern edge of China’s Kumtagh Desert, connects to the desert through the Aqik Valley. The dunes are tightly linked to the desert through complex sediment transport processes and landforms. To clarify regional geomorphological processes and the source regions for the dunes, we conducted long-term field studies of sediment transport, analyzed the spatial distribution of grain-size characteristics, and their variations with wind direction and height. The transported particles are mainly (73.3%) fine and medium sands, moderately to well-sorted, positively skewed to fine, with mean grain sizes of 1.12–2.19 Φ (220–460 μm). The finest, worst-sorted particles occurred near the surface (0–10 cm), whereas the coarsest and best-sorted particles at the top layer (40–100 cm) were found in sediments transported from the prevailing wind direction (NE); the finest, worst-sorted particles were found at the top layer, whereas the coarsest and best-sorted were found near the surface for sediments transported from the weakest wind direction (S). We have found multiple patterns of vertical variation of the grain-size parameters. With the increase height, transported particles become coarser, better-sorted, and the frequency curves narrowed, nearly symmetrical to positively distribution in prevailing wind direction, while becoming finer in S wind direction. The grain-size distribution of transported particles in different directions and heights are mainly dominated by wind intensity, Gobi surface properties, and grain size in this region.