Historically,many paleochannels formed along the current main stream of Tarim River due to frequent migrations.Research on the grain size characteristics in surface sediment of the paleochannels is helpful to determine the sedimentary environments and hydrodynamic conditions during the surface sediment formation.Through systematic samplings and grain size analysis for surface sediment of the paleochannels,we find that:(1) silt is the dominated grain fraction in surface sediment of paleochannels and the secondary grain composition is obviously different between the south and north paleochannels along the current main stream;(2) the grain size for surface sediment of the paleochannels is smaller than that for surface soils in the floodplain of Tarim River,which suggests that hydrodynamics was relatively weak in the late period of paleochannels formation;(3) there are eolian sand layers in surface sediment of some paleochannels on the south bank of Tarim River,which implies that the sedimentary environments were influenced concurrently by wind and water.