Real-time observational data including 500hpa charts and surface charts from 1995 to 2000 after being analyzed indicate the dust storms caused by cold air crossing the mountains in Xinjiang were classified into two types according to the paths which they had taken: the westerly path type and the northerly path type. The synoptic causes and forecasting indexes of these two different types were pointed out combing research results and forecasting practical experiences. And a typical case of the westerly path type occurred during May 4-6 in 2000 was given to show the weather process, synoptic causes and forecasting indexes. Also the analysis of three typical dust storms of the northerly path type occurred during July of 1996 reveals how cold air crossed the mountains down to the Taklimkan Desert and caused dust storms in summer. The common opinion is refuted that in summer cold air is weak and severe dust storms caused by cold air crossing the mountains cannot occur.