Based on the encryption sounding data, the automatic meteorological observing data and other conventional observing data, we analyzed a severe sandstorm occurring in the Midwest Inner Mongolia autonomous region on 23—24 April 2009.The ground meteorological elements and its internal dynamical and thermal changes in the process were studied.The result showed that:1)Before the sandstorm, the temperature lapse rate was big, the relative humidity was large, the wind speed near the ground was small and the wind direction was easterly;when the dust storm occurred, there was an inversion in the atmosphere temperature, the relative humidity was small, the wind speed increased obviously and the wind direction was westerly and stable;when the sandstorm ended, the temperature decreased, the relative humidity increased fast and the wind velocity reduced rapidly in all layers;2)The atmospheric stratification had been conditionally unstable before the sandstorm took place and turned to be stable in the low layer and neutral between 800 hPa and 600 hPa when the dust storm occurred, and the low layer atmosphere was extremely dry at the time.Then the humidity became large when the sandstorm ended.So "3θ"curve could provide a valuable reference for the monitoring and forecast of dust storms;3) The relative storm helicity(RSH), which was available when the dust storm occurred, was of the maximum value compared with all the other observation times, and it had a good correlation with the sandstorms in intensity.That was to say, the bigger the RSH was, the severer the sandstorm would be;4) The intensity of the sandstorm was not only closely related to the surrounding velocity, but also to the values of the tangential and vertical velocities in the sandstorm.