Abstract: In loess area, both artificial rainfall field test and in-situ observation under natural rainfall conditions suggest that the influence of surface water is limited to the top several meters, below which the variation of soil water content is little. Therefore, it is a controversy to fact that the surface water infiltration can induce loess landslides with deep seated failure surfaces. In this study, an in-situ observation test is undertaken in a thick loess layer for a period of one year from June 1^st, 2015 to May 31^st, 2016. Results show that water infiltration in the loess can be divided into three zones, namely, active, stable, and saturated zones. The active zone is influenced significantly by precipitation and evaporation events, where water flow is a transient behavior. The stable zone is characterized with a stable water content and steady seepage flow. The saturated zone is where groundwater exits, which can be drained in the forms of spring. In the long run, the hydraulic condition reaches an equilibrium state with a stable water content in the stable zone and stable ground water table level. However, this equilibrium state can be disturbed with a higher surface water infiltration, for example irrigation water and leakage from pipelines, which contributes to a new equilibrium state with higher stable water content in stable zone and higher GWT level. Loess landslides with deep seated failure surfaces may occur during the transitional process from the initial equilibrium state to the new equilibrium state. Yanlian deep loess landslide is taken as an example to illustrate the mechanism of surface water infiltration induced deep landslides. The hydraulic response to long-term surface drop water and its influence to slope stability are analyzed. Results highlight the close relationship between deep loess landslides and long-term surface water infiltration, which is easy to be neglected due to its slow flow rate.