Abstract: Lacustrine low liquid limit clay layer,formed by downstream dammed lake,distributes in overburden layer of Shangjiang dam on the middle reaches of the Jinsha River,which is a relatively weak area of the dam foundation with the characters of the depth of 49.7～79.6m,the thickness of 9.3～24.0m,thick on the left bank and thin on the right bank,continuous distribution,high plasticity,low strength and water permeability.The river bed overburden layer is deep in the dam site,with the depth of 206.2m,where distribute low liquid limit clay layer but also drift gravel,boulders and gravel,stone fragments,gravel and so on,which have relatively high strength.It is planed to set core rockfill dam at the beginning in order to take the overburden layer as the bearing layer.For the choice of dam site is in close relations to the clay layer,the paper analyses on the spatial distribution,cause of formation and engineering properties of the bed low liquid limit clay,and studies on the dam technical problems of using the clay layer or not,finally proposes the economic and reasonable program of using the shallow layer of clay as the dam foundation.