The weather circulation background and dynamic mechanisms for a sandstorm case occurred in the western Inner Mongolia on 23 April 2009 are analyzed by using observation data. The results show that the main cause for this sandstorm case is strong cold air activities. The upperlevel system causing the sandstorm is the deep Mongolian transverse trough and northerly airflow on the east; the surface system is the strongly developing Mongolian cyclone. From the view point of dynamics, when the sandstorm happened, the momentum of northerly airflow aloft was transmitted downward to surface with prevailing descending motion because of convergence at upper levels and divergence at lower levels. The secondary circulation generated by disturbance development at middle and low levels made momentum transmit downward effectively. It is worthy to note that the key points for the storm are the southward movement of the upper Mongolian transverse trough and its rotation to upright position, which caused clod air to break and move southward; the upper northerly jet and the obvious development of the surface Mongolian cyclone around the exit of the jet were the necessary conditions for sandstorm generation; the continuous reinforcement of pressure gradient after the surface cold front speeding up intensified the burst of clod air; and the strong convergence aloft and divergence below increased the possibility for sandstorm generation.