Two dust devil events in Beijing area on 6 and 9 April 2000 are analyzed. The result shows that the two events caused by the Mongolia cyclone and the northwest trough without cyclone are different. In the area of sand source, the strong ascending movement at the middle and low level lift the sand and dust to sky vertically, then the strong northwest airstreams at 700hPa transport them to the downriver area flatly; the instable atmospheric structure could strengthen the dust devil; the distribution feature of z-helicity of the dust devil area is negative at the high level and positive at the lower, and the positive area is coincident with the satellite image.