‘7·21’ rainstorm has been the new record of precipitation process in Beijing. Before the rainstorm, Tongzhou district in Beijing's eastern plains suffered strong wind attack. Because of not witnessed, whether it is caused by a tornado or not has been the focus of debating. After studying synoptic environment and radar echo data, it is proved that it is a tornado caused the gale; then using VDRAS data analyzed the dynamic structure of supercell storm. Study showed, the synoptic situation, vertical structure on temperature, humidity and wind are very conducive to the occurrence and development of tornadoes. BWER in reflectivity echo and mesocyclone in velocity echo showed it's a supercell storm; the convergence in 300 m and TVS further confirmed it's a tornado process. At last using the VDRAS inversion wind field showed the dynamic characteristics of the supercell storm: Low level in right rear the storm movement direction is easterly inflow layer, in start stage, easterly inflow layer is in higher, then the easterly wind weakened with height reduced. It forms a stable wind shear with high level enhanced westerly wind of outflow layer; in the mature stage, the storm has strong easterly inflow in low-level, and westerly outflow in high-level. At the core of supercell storm there is strong upward movement, so the supercell storm itself has forming a complete vertical circulation. The supercell storm is weakened when the environmental vertical wind shear is weakened and vertical circulation collapse.
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