Abstract：The paper analyzes the grain-size distribution of riverbed alluvial deposits in Harbin, and on that basis, combined with the grain-size distribution of sand-dust fallouts, it discusses the impact of bare riverbed alluvial deposits on the sand-dust weather. The analytic results show the following：the >63μm particles of bare riverbed alluvial deposits constitute >84% of the components, while the <63 μm particles are rare and those <10μm are very rare; the content of silt-clay particles <63μm amounts to >90%. Both sand-sized coarse grains and silt-sized fine grains of riverbed alluvial deposits show no correlation with the grain size of sand-dust deposits and have no contribution or little contribution to the material composition of sand-dust storms. Restricted by the flying height and transporting distance of sand-sized (>63μm) coarse particles, particles of alluvial deposits of the bare riverbed cannot be transported over long distances when sand-dust storms happen; so they have little influence on the whole Harbin area. The sand-dust materials that really influence the whole Harbin area are silt-sized (< 63μm) particles, especially dust (<10μm). It is considered that what have major effects on the sand-dust weather in Harbin are surface soils and building soils in the city that contain voluminous fine particle materials. These sites are key areas that should be controlled for preventing the sand-dust weather in Harbin City.