Barchan sand dunes, which are distributed at the oasis fringe, are an important type of dune. The study of the mechanism of stability of barchan sand dunes is of great importance to the determination of the regulation of aeolian-sand movement at the oasis fringe. However, the stability of barchan sand dunes remains an unresolved scientific issue. This study took the barchan dunes in the Hexi Desert as a case study. The stability mechanism of the top of barchan dunes was analyzed. The results showed that: (1) The barchan dunes in the Hexi Desert of Gansu Province were distributed as patches downwind from the desert fringe. The distribution area was flat sandy-clay or sandy-gravel soil. The lowlands among the dunes were wide, with prominent prevailing winds. (2) The prevailing northwestern winds caused the top of the dunes and the sand line to coincide, and the dunes to move forward and become lower. The southeastern winds (opposite to prevailing winds) caused the top of the dune to separate from the sand line, and the upper part of the leeward slope was wind-eroded and the dunes were heightened. (3) The higher the barchan dune was, the longer the two flanks were, and the smaller the angle was. (4) The wide flat area was vital to the formation of barchans and barchans chains. The largely prevailing wind was the key factor for the stability of the barchans and barchans chains. The opposite wind and the corridor effect between the two flanks were key factors enabling the barchans to grow in size and height. The wind speed and distribution frequency of the prevailing and opposite winds were factors restraining the height of the barchan dunes and barchan dune chains.