Abstract: Dust storm is a weather phenomenon over the desertification area. Nature factors tie together with human activities that have been put forward to account for the formation of dust storm. Nature factors include high wind, decreasing precipitation and enough sand sources. Human activities result in increasing of dust storm by decreasing vegetation covers in semiarid and arid regions during the economical development. Dust storm has been a serious climate disaster for northwest and north China, and the studies about dust storm focus on the blown sand dust and sinking of sand-dust. Many studies prefer the former in theory, so the work on the sinking is necessary. Tiny dust particles whose Stockes terminal velocity can be overlooked will move with the air, and the air and the dust form a two phase flow. We discussed the dust's sedimentation condition and velocity under mean and more real environment by two-phase flow. Under the mean environment the advantageous initial conditions for sinking are a strong disturbance of initial dust concentration and a weak disturbance of initial air parcel's temperature. further work is on the more real environment, and the model gives three characteristic sinking areas: (1) fast sinking area, where the sanddust sinks fast; (2) slowly sinking area, where the sanddust will rise first, then it sinks to the ground and the time for the total sinking is long; (3) wave sinking area, where the sand-dust will wave in the environment and it is hard to determine when sand-dust will sink to the ground, so the time for sinking is determined by the condition of the ground. The conditions for the sand dust's continuous rise are also discussed finally.