Abstract: In order to determine the failure mode of slope sliding in the southwest side of Weijiamao open pit mine, the position of slope sliding surface was determined, the failure evolution mechanism and failure mode of slope rock mass were revealed ,the failure mode of the slope was found out and the control plan of the landslide was put forward through the field and geological survey of sliding mass, deep and surface displacement of slope joint monitoring. The results show that the potential slip surface of the slope was 27m-32m below the orifice; the displacement of the slope present a "L" type change process of "rapid growth- slow balance- readjustment", and the slope was in limit slip state at present; the slope slip presents a mechanical mechanism of "excavation unloading shear failure", and the macro failure mode of the slope was shear failure; the weak interlayer was eroded by water and the stress of rock mass was released peeled off after advancing was the main reason of the slope sliding.