Under the slope failure criterion based on the breakthrough of the plastic zone, the mutation of displacement increment of slope and calculation convergence, strength reduction FEM and gravity increase FEM are used to analyze the stability of simple slope. Studies indicate that it is accurate to regard the mutation of displacement increment of slope as slope failure criterion; slope safety factor for different strength parameters based on the criterion of mutation of displacement increment is more stable than based on the other two failure criterion. According to three failure criteria, the breakthrough of the plastic zone, the mutation of displacement increment of slope and calculation convergence, the potential slip surface is in turn to develop in depth, and the safety factor of the slope is increased in turn.