Abstract: Based on the fact that forces are vectors, the key problems of the vector sum method are the state of real stress distribution and potential sliding direction of slopes. In a slope under certain conditions, after the determination of stress distribution by FEM, different safety factors can be obtained by the vector sum method with different potential sliding directions of the slope. Three definations of the potential sliding direction of the slope are put forward, and then the fixed circular slip surfaces, non-fixed circular slip surfaces and circular slip surface searching are discussed respectively. It is shown that the potential sliding direction defined by anti-sliding capacity of the potential slip surface is the most reasonable of the three. Finally unified formula about the VF in 2D and 3D problems is presented. Its calculating process is very simple and it is very easy to be applied in practical engineering problems. So the vector sum method can be widely used in practice.