Abstract: In engineering practice,the safety of slope is described generally by the stability safety coefficient.Each coefficient is corresponding to specific sliding boundary. In order to evaluate the stability of slope rightly,the most dangerous slip surface must be exactly determined in advance. Factors such as excavation,load and reinforcement affect engineering slope stability greatly.They often cause the change in the most dangerous slip surface,make evaluation difficult and even 1ead to wrong conclusion.In this paper,based on engineering practice,factors caused the phenomena, including structure type of slope,resisting action of the possible slip surface,loading, excavation, hydrogeology condition,prestressed reinforcement and bulkhead support. And then take Jiangyin Bridge Project as an example,effects of loading and excavation on the dangerous slip surface are discussed.The FEM calculation results based on interference displacement showed that. Under the action of vertical load of 1.2×10 6kN ,the most dangerous pasive slip surface of south tower foundation slope moved 30 meters downward.Excavation made the most dangerous slip surface of the man made slope in south tower zone moved 6 meters downward. The FEM calculation of water inlet slope in Xiaolangdi Project showed that engineering excavation made the most dangerous slip surface moved from deep to surface.