Abstract: The action on water slope includes surface water flow and surface water infiltration, and surface water flow, such as slope damage of reservoir and river banks, is caused by hydrodynamic erosion. At present, there are some vague concepts in slope stability analysis under the influence of surface water infiltration in loess. For example, ignoring the stress path in the infiltration process and only considering the stress state at the time of its failure will lead to misjudgment of its failure mechanism and the value of stability calculation parameters. This paper aims to analyses and clarify the issues. There are two sources of groundwater in loess, those are rainfall and irrigation. Rainfall infiltration produces shallow slope failure during raining time, while irrigation causes groundwater level rising to trigger deep seated slide. Surface water infiltration can make a remarkable rising of pore water pressure, but minor change of the total stress, and a consequent decreasing of effective stress in the slope, which is the cause of slope failure triggered by water. The initial pore water pressure in unsaturated loess is negative, and the shallow loess after rainfall infiltration is still in unsaturated state, and the pore water pressure rises to 0 at the maximum. Irrigation will cause the groundwater level to rise, and the pore water pressure under the water table will be positive. When the change process of pore water pressure is made clear, the slope stability can be evaluated by effective strength. At the same time, at present, some opinions think that the landslide of flowing loess is caused by static state, which reverses the causality, and it is the slip that causes liquefaction, not the slip caused by liquefaction.