Abstract: There aremany outw ash deposits distributed in the m iddle- e levation area of them idd le reaches of the Dadu river. Ow ing to the dense structure o f the outw ash deposits, and the detritus is cemented in some deg rees sometimes. So the deform ation and fa iluremechan ism o f the outw ash deposits under scour ing o f riverw ater flow are different from those of other deposits. This paper invest igates the outw ash deposits on a reservo ir bank and tests them. A conclusion is obtained that the outw ash deposits should be classified as m ixture so il w ith coarse granule and g iant granule. The granules w ere tabled densely, and the ou tw ash deposit intensity is higher than o ther depos its. B ased on the findings, a detailed study is g iven on the deform ation and fa ilure mechan ism of the outw ash de posits under act ion o f riverw ater flow by them eans of geo log ica l process analysis and numerical simulat ion analysis. It is figured out that the outw ash deposits shou ld be fa iled like eroding- co llapsing under riverw ater action.