Abstract: High or super-high bench dump′s granular material block size distribution show obvious size grading characteristics，and its strength characteristics vary remarkably with pieces of cloth.The indoor direct shear tests reveal the rule of dump granular material strength parameters C、φ and block composition.On this basis，the strength reduction method is adopted to study the variation trend of the potential slip surface of slope and the safety coefficient at strength parameters varying with the height，through layering the dumps.The experimental research and the numerical simulation show that:the internal friction angle increases with coarse particle content increasing，showing as a linear relationship.Its cohesion varies as a curve，and its overall variation trend becomes more and more slight.The potential slip surface of slope is continuously sub-divided with the granular material layer.The outlet of potential sliding surface is moving upward，and the plastic zone is slightly expanding to be a final steady state; The safety coefficient decreases gradually with the increase of layer number.When the decrease reached a certain value（about 1.125)，the safety coefficient emerges a rising trend，but the trend is becoming stable with the increase of layers number.

Key words: Strength property, The potential sliding surface, Strength reduction method, Safety factor