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Barchan dunes, an important type of dunes, are distributed on the edge of oases. Research of stability mechanisms of barchan dunes is of great importance to the determination of regulation of aeolian sand movement on the edge of oases. However, the stability of barchan dunes remains an unresolved scientific issue. In this paper, the stability mechanisms of the top of barchan dunes in Hexi desert area of Gansu Province were analyzed. The results showed that: (1) in Hexi desert area of Gansu Province, patches of barchan dunes were distributed downwind on the edge of desert. The distribution areas were flat sandy-clay or sandy-gravel beaches. The lowlands among the dunes were wide and had the prominent prevailing wind. (2) The prevailing northwestern wind caused the highest point and sand ridge line of the dunes to coincide, and the dunes to move forward and become lower. The southeastern wind (opposite to the prevailing wind) caused the highest point and sand ridge line of the dunes to separate, the leeward slope to be eroded by wind and the dunes to rise. (3) The higher the barchan dunes were, the longer the two flanks were, and the smaller the angle was. (4) The wide flat beaches were vital to the formation of barchan dunes and a chain of barchan dunes. The largely prevailing wind was the key factor for the stability of barchan dunes and a chain of barchan dunes. The opposite wind and the corridor effect of the two flanks were key factors for the increase of barchan dunes in size and height. The speed and distribution frequency of the prevailing and opposite winds were factors restraining the height of barchan dunes and a chain of barchan dunes here.