A strong sandstorm weather process occurred on the 12th April 2000 in Gansu,Shanxi and Ningxia provinces is analyzed.The results show that the event was caused by the northwest strong cold air.Before the occurrence of sandstorm,continual temperature rising was the favorable thermal condition,and the development of Hexi cyclonic eddy was meso scale system which stimulated the sandstorm.The development and persistence of dry warmer sector in northwest is the index of its occurrence.Unbalanced state of the abnormal jet intensified the front over middle and lower troposphere and the unsteady atmospheric level which provided a favorable large scale circulation environment.Moreover,the long and narrow terrain and special quicksand,quick ore sand over the ground surface also provided a favorable condition geographically.