A case study is made of the dust storm occurred on 19, 20 April 2005 in the southern Hebei Province from the aspects of the climatic background, synoptic features, orographic effect and dynamical and thermodynamic mechanisms. The results indicate that before the occurrence of the storm, the local weak convergence, upward motion and unstable vertical structure drew local dusts up; the ups tream strong upward motion drew dusts up and then the upperair strong northwesterly transported the dusts to the downstream area horizontally; the dust storm occurred at the downstream area of the maximum positive helicity zone in the troposphere. The magnitude of the helicity in the troposphere is useful for the prediction of dust storms.