Abstract: Due to the long-term infiltration effect of agricultural irrigation water, more than 130 landslides have been found since 1984 in the Heifangtai loess tableland, Gansu Province. Among them, more than 90% of landslides can be classified into the homogeneous loess ones. The formation mechanism is unique. Generally, the irrigation water can permeate the loess body and gather at the bottom of loess layer to form a saturated weak base with more than 20 m in thickness. The corresponding shear strength and compressive strength of this base layer are reduced greatly, while the deformation capacity increases. Under the loading impact of overburden, the saturated loess base layer will exhibit the phenomenon of severe plastic deformation towards the outside of the slope. Moreover, the relatively large lateral pressure can be generated in the saturated loess area at the back of the landslide and horizontally act on the front slope to cause deformation damage, and then results in the high-speed slipping, which is another one of the power sources to induce the saturated loess landslide. The observated data from the in-situ investigation show that the lateral pressure of soils is mainly generated in the saturated loess layer below 23.5 m, and it can reach 30~230 kPa in Heifangtai loess terrace area. The horizontal thrust may increase with the growth of depth, and can even reach as high as 2236 kN/m. In the case of the Luojiapo landslide which occurred on 29^th, April, 2015, its the stability coefficient is 1.12 when the lateral pressure is not taken into account, while it is reduced to 0.94 due to the effect of lateral pressure. Therefore, it can be concluded that the lateral pressure generated in the saturated loess layer at the back of landslide plays an important role in the formation of saturated loess landslide.