The forming mechanism and the model for pyramid dune formation were not fully studied. In this paper. the forming conditions of pyramid dunes are analysed, and a regional model is estab-lished based on the observation of airflow and the experiment in wind-tunnel. The forming conditions of pyramid dune include;a rich sand source and a wind regime which contains three or more directions which have similar windforces. The mountain-valley wind resulted from local landforms is one of main factors that cause the changable wind directions. The forming mechanism of Pyramid dune can be inferred from the air flow analysis and the experiment in wind-tunnel. Under the changeable wind directions in one season, a barchan appears firstly; then, the embryo-pyramid dune emerges, which embodys three slip-faces and three tounge-shaped protrudings between them. With the three-direction winds modifying alternatively from season to season, a crossaxis eddy result in the lee-side and inclined helical eddies to form at the slope turn, which take sands from lee-side to slope turn and transport some of them upslope simultanneously. Finally, mature pyramid dune arises, which is made up of a pyramid top, three sharp ridges and three concave slopes. Sometimes, the pyramid dune may break down and become a barchan when the wind blows toward one of its slopes.continously for a very long period.