Based on conventional observation data and ECMWF numerical forecast products, dynamic diagnostic analysis was carried out to find out the causes for the formation and weakening of sandstorms that happened in the Hexi Corridor of Gansu Province on 23 April 2014. The results showed that unstable trough develops strongly in front of the ridge of Mountain Ural at 500 hPa, steering strong cold air from the North Pole to move southward quickly, which is the largescale system leading to the sandstorm. And strong storms accompanying the surface cold front is the main cause for the outbreak of the severe sandstorm. The reason why the strong sandstorm gets enhanced in the Midwest of the Hexi Corridor is that nearly neutral temperature stratification in 700-850 hPa coupled with strong cold temperature advection and lowlevel jet of Hexi Corridor is conducive to the frontogenesis and downstream of momentum. The strong pressure gradient, temperature gradient and daily variation before and after the cold front causes explosive development of sandstorm in the eastern part of Hexi Corridor. In addition, the strong upward motion of low altitude near the Midwest of Hexi Corridor is in favor of increasing dust concentration. Strong differential temperature advection and vertical wind shear enhance thermal and dynamic instability. The reason why strong sandstorm weakens in eastern part of the Hexi Corridor is that the main weather system resulting in the strong sandstorm weakens and moves northerly, and the narrow tube effect disappears in the eastern part of Hexi Corridor. Besides, the diurnal change of temperature and pressure gradients at midnight lead to frontolysis. Even the rainy weather continues in the early stage so that the exposed surface soil becomes loose and deserts form a compacted layer to increase the difficulty of raising sand.