Abstract: Cut slopes are usually formed during excavating in rock mass. The rock structural surfaces are exposed and rock blocks are formed. They influence the cut slope and cause it breaking. The failure would occur after the original static equilibrium state is destroyed and the stress is redistributed. Therefore,the stability of rock blocks formed with structural surfaces is the important factor that determines the stability of rock slope. This paper establishes a 3D solid model to realize the visible operation of cut slope and structural planes and excavation terrain model on an AutoCAD Platform. It then makes sectional cutting of twodimensional graphs in different locations. It further makes a forecast and analysis of structural surface assembly blocks possibly existed after excavation. It adopts the block computational procedure(Swedge)to verify the key block and makes an evaluation on its stability. The results show that its easily to search the key block of rock slope through 3D solid model and the slice technology in combination with block calculation. It is proved that this method is simple. The calculation result is accurate. The search method is feasible．