Abstract: The mechanism of irrigation-induced loess landslide is one of the top topics of loess geo-hazard researches. Furthermore, the key areas are the hydraulic and mechanical property changes of loess after drying and watering alternation medium under irrigation condition, as well as the slope stability changes due to groundwater flow field response. This paper discusses the developing mechanism of irrigation-induced loess landslide in Heifangtai, which is on the basis of the hydraulic and mechanical property tests of unsaturated loess in Heifangtai, the unsaturated seepage evolving process and developing trend of the slope area due to irrigation. The results demonstrate the following. With the lasting of irrigation, long-term positive water equilibrium field can trigger the rising of the groundwater table. Moistening of the unsaturated zone leads the matric suction to decline and the strength to reduce drastically. Meanwhile, the thickness of the saturated zone increases. This causes build-up of porewater pressure as well as augmentation of the hydraulic gradient in the slope. The seepage velocity fastens. Hence the seepage force on the slope escalates too. Then the slope stability drops. The factor of slope stability declines linearly with the rising of groundwater table. The slope reaches limit equilibrium state when the groundwater table is close to 55m, under which condition it may slide under certain negative triggering factors. The groundwater table change, however, has limited impact on the critical sliding surface.