In the years 2002 and 2003,the frequency of the sandstorm in the middle and west region of Gansu decreased and the intensity weakened.In order to find the Climatological reason for the change,the arctic vortex,temperature and the moisture of surface layer soil were analyzed.According to the source of sanddust,the transfer paths of sanddust can be devided into two type:the northwest and west path.The composite analysis is used to find their mean circulation,and in order to find the relation between severe sanddust storm and physical quantities,such as Q-vector,CSI(Conditional Symmetric Instability)and frontal secondary circulation,a severe sandstorm "20010408" was diagnosed based on the NECP data.And the result show that conditional symmetric instability layer was very thick before the happening of the sand storm and the frontal secondary circulation had a close relation with the sand storm.