Abstract: Due to long-term irrigation, dozens of loess landslides were triggered along the edges of Heifangtai plateau, Gansu Province. Long-term infiltration of irrigation water not only leads to the elevation of groundwater table but also results in the reduction of strength of water-sensitive loess after soaking, and thus loess landslides occur. As the shearing mouths of the loess landslides are usually situated in the middle of the slopes which are gentle on the top and steep at the bottom, the disaster modes show features of high elevation shearing failure and later high-speed sliding. The landslides usually have long run-out, which affect large areas and cause severe consequences. A landslide case in Jiaojiayatou of Heifangtai in Gansu Province was studied in this paper. Based on field investigation and groundwater in-situ monitoring, the authors analyzed the formation mechanism of the plastic flow-tension crack mode of irrigation-induced loess landslide. Discrete element method was used to investigate the formation and evolution process of weak saturated loess layer in the process of the rising of groundwater table. The results show that, with the rise of the groundwater table, areas of the weak layer at the bottom of the model continuously increase. The pore water pressures mainly act on the weak saturated loess at the bottom of the landslide. When the groundwater table rises to an elevation of 1682m, overall failure of landslide occurs.