The sand and dust weather that affected Beijing on 27-28 April 2012 is studied. The finding suggests that the sandstorm in sand source area happens mainly behind the cold front and the 500-hPa trough, where, because of the strong surface sensible heating and the cold advection in middle and lower troposphere, the deep mixed layer or even the unstable stratification forms and can reach up to 600 hPa, and such instability of stratification is an important condition for the sandstorm to affect the downstream area. Moreover, based on the analysis of the variation of the mixed layer thickness and its advective process we also study the transportation pathways and locate the deposition area of the sandstorm. It is found that there are two transportation pathways. One extends eastwards along the northern part of Liaoning Province and then turns to northeast with a higher height. Dust particles move downstream with the advection of the mixed layer. Afterwards, with the decreasing height of the mixed layer, the deposition occurs and sand particles reach the surface. The other extends southeastwards along the south of Liaoning Province. At the stage of transportation, the advection of the mixed layer is clear, but the mechanism of the deposition is different from the former one. The deposition occurs due to the descent in lower troposphereand the surface divergence. Afterwards, along the easterlies on the south side of the cold anticyclone behind the cold front, the sand particles are transported westwards to Beijing. In addition, the latter path is lower than the former because the stable layer which lies above the mixed layer descends obviously. Based on HYSPLIT model the backward and forward trajectories are computed. It proves the above two transportation pathways, showing that it is the easterly return flow that results in the sand dust event in Beijing.