Knowledge of dust sources is crucial for understanding the aeolian system of the Xiashu loess which is distributed in the middle and lower reaches of the Yangtze River and in the Huai River region, and provides a unique long-term record of the environmental evolution of the humid subtropical region of East Asia. The debate on the dust source of the Xiashu loess arises partly from the lack of spatial information on its source signatures. In this study, we investigated spatial changes in the grain-size distributions of samples from 20 Xiashu and 6 Chinese Loess Plateau (CLP) loess sections of the Last Glacial period. The aims were to determine the genetic relationship between the Xiashu and the CLP loess, and to determine the transport pathway of the Xiashu loess. The grain-size data reveal that the trends in fining and sorting of the Xiashu loess do not conform to its being an extension of the CLP loess. This conflicts with the early view that the Xiashu loess represents a southward extension of the CLP loess. The southward-fining trend indicates that the Xiashu loess was mainly transported from north-to-south, and that the prevailing aeolian transport agent is the East Asian winter monsoon. We also examined present and published records of the transport pathway of the Xiashu loess and concluded that the deposits are mainly linked to the adjacent dessicated fluvial system of the Huai River. Thus, the Xiashu loess is mainly a repository of information on the evolution of this adjacent fluvial system.