Some desert plants can accumulate sand into mounds and form nabkhas, whereas some cannot. The reason for the difference is unknown. To understand the difference, we selected several common sand-fixing plants in the Minqin desert area and collected 108 samples of these plants and sand from the mounds along the fringe of Minqin Xishawo. We measured and compared the layered silhouette width, the layered silhouette area, and the silhouette center of different plants. The results showed 1) the silhouette of plants that form sand mounds are triangular or cylindrical, which means the width of the silhouette becomes smaller from the ground to the top, and the silhouette center is within 30 cm from the ground. Plants with this characteristic are clumping shrubs. The silhouettes of plants that do not form sand mounds were diamond-shaped, and the silhouette center was 30 cm or more from the ground. 2) Sand particles in aeolian-sand flow and sandstorm flow were primarily concentrated within 0-30 cm above the surface, which was an important reason why the lower the plant silhouette, the more easily the nabkhas were formed. 3) Plants with stems that can produce adventitious roots after sand burial can form sand mounds or dunes, which are several to dozens of times higher than the plant itself. Although some plants do not produce adventitious root after sand burial, they can nonetheless form sand mounds or dunes; however, the height of the sand mound or dune will not be higher than the plant itself because of the limitation of the height of the plant itself. 4) The size of the silhouette was an important indicator that reflected its function as a windbreak and sand-fixing properties of the plant. The plant's ability to accumulate sand into a mound was the manifestation of the function of stopping and fixing mobile sand. Higher sand mounds reflected a stronger function in stopping and fixing mobile sand by the plant. However, plants that do not form sand mounds or dunes, such as Haloxylon ammodendron, Calligonum mongolicum and Artemisia arenaria etc., also can function in stopping and fixing mobile sands.