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In order to ascertain the formation cause and development mechanism of mega-dunes situated in the south-eastern Badain Jaran Desert, emblematic mega-dunes in densely distributed area was chosen as research object, and combined grain-size distributions with physiognomy zonation of a single mega-dune to carry out systematic and quantitative research. We acquired the physiognomy zonation data on the windward slope, the sediment size composition at different parts of the mega-dune by means of grain-size analysis and further investigated the relationship among sediment size distribution, content change and the landform zonation characteristics of the mega-dune's wind dynamic conditions. The following conclusions were drawn: (1) The grain size of sediments in the PSW (i.e. the surface profile of the mega-dune's windward slope) is coarser than that in the PSSW (i.e. the subsurface profile of the mega-dune's windward slope, about 10 cm under the surface) and PSL (i.e. the surface profile of the mega-dune's leeward slope), and its average kinetic energy of transportation is also maximal. (2) According to the observational data of the secondary geomorphology and grain-size distributions of sediments, the windward slope can be divided into four main zones from the bottom up: a low-lying sandy zone, a sparse barchan and dune chain zone, a dense barchan and dune chain zone and a steep main peak zone. The grain-size distributions show trends of becoming finer from bottom to top, which is determined by the formation progress and the dynamic action differences. (3) The sand materials at the bottom of the windward slope can reach the top of the mega-dune or cross the windward slope, subsiding and accumulating on the leeward, through many occurrences of aeolian transportation.