A special study on the interaction mechanism between flow and soil is of great significance for revealing the macro breaching mechanism of barrier dams. To study the scouring characteristics of wide-graded sediment under different flow conditions, flume scour tests were conducted regarding the grading curve of dam material and the discharge process of the Tangjiashan barrier dam. The results show that: (1) The scouring process of narrow-graded or uniform sediments is the formation and movement of the sand wave, while the scouring mode of wide-graded sediment is mainly the formation, expansion, and movement of the scouring pit. (2) Under the condition of weak unsteady flow, the surrounding and shielding effect of coarse particles on fine particles is obvious, and the erosion resistance of the material is strong. However, under the condition of strong unsteady flow, the erosion resistance is weak. (3) The erosion of wide-graded sediment is mainly caused by slope angle collapse at the initial stage, and mainly reflected by traceable erosion at the later stage. Therefore, in the initial stage of erosion, the downstream erosion intensity is high, and the bed surface can easily form a slope inclined downstream. (4) The scouring intensity under the condition of unsteady flow is greater than that under the condition of steady flow. The sediment transport formula based on the condition of steady flow cannot be used to calculate the dam break process directly.