The northern margin of Hobq Desert is strictly limited to the south of the Yellow River and the river in arc north basically is the northern border of the west section of Hobq Desert. Hobq Desert developing east—west 19 ka ago has always been extending northward since 7 ka ago. By analyzing data such as historical document, remote sensing image and geomorphic sedimentary etc., this paper holds that the desert from south to north led to the change of Yellow River from extension east—west to bulge in an arc, and caused the situation that Hobq Desert is strictly limited to the south of the Yellow River. Hobq Desert's northwest expansion will have a serious threat to the ecological environment safety of wetland to the north of Yellow River in Hetao basin.