The identification of sediment transport processes is essential in understanding the accumulation and development of sand seas. However, only limited effort has been made to extract macroscale sediment transport information in aeolian science. In this study, we investigated grain-size variation to gain insight into large-scale sediment transport processes in the Kumtagh Sand Sea. Results suggest that grain-size variation is correlated to fluvial-aeolian interaction. In the central sand sea region, due to intermixed fluvial-aeolian controls, the downwind trend of grain size was significant except for the area surrounding the channel belt; but the grain-size trend in a downstream direction was not obvious. In the northeastern sand sea region, dune sand became progressively finer along a downwind direction, but sorting remained consistent. This phenomenon is ascribed to the fact that sand from upwind areas is mixed with local interdune sand. By combining evidence from dune patterns, grain sizes and mineral composition differences, we determined that sediment transport processes in the Kumtagh Sand Sea can be summarized as follows: Fluvial system transport sediments from the Altyn-Tagh Mountains to the sand sea, after which prevailing northerly winds subsequently rework these sediments and deliver them to downwind areas; meanwhile, sand from upwind areas is mixed with local sand in the northeastern sand sea region. Our findings provide insight into modern sand sea formation under the control of fluvial-aeolian interactions, while potentially offering further implications that can be used to explore paleogeographic process based on these modern processes.