Plagioclinal slopes in cuesta escarpments are oblique to bedding dips and widely distributed in the mountainous area of southwestern China, and the failure patterns are conventionally assumed to be rockfall or slab collapse, rather than rockslide. The catastrophe of the Jiweishan rockslide on June 5, 2009, illustrates a unique mechanism of plagioclinal slope failure, where the slope failure moves in the direction of apparent dip and is confined by four planes. Geological analysis concluded that the Jiweishan rockslide was a result of the weak-interlayer-controlled geological structure, underground mining, and karst. Modelling tests reproduced the instant failure of key blocks in the apparent dip direction followed by block glide, and they suggested that weakening of the slip surface and the discrete structure contributed to slope failure. Numerical simulation indicated that rear blocks acted on relatively stable key blocks during long-term gravitational creep on the weak interlayer. Strength reduction of the weak interlayer was a key factor in the rockslide occurrence. Ultimately, a three-dimensional limit equilibrium method of stability analysis for apparent dip slide is proposed, along with validation of modelling tests.