Abstract: Loess collapse-type lateral pressure have a landslide-promoting effect in loess slope areas, which is a basic scientific problem in the study of loess landslides induced by irrigation. A study on collapse tests and slope stability simulations of loess were conducted. The variability of loess collapse-type lateral pressure in the Heifangtai area and its sliding mechanism has been analyzed. The results indicate that Heifangtai loess is self-weighted collapsible loess; soil of the irrigated area at depths of 0-15 m are slightly collapsible; soil at depths of 20-25 m are moderately collapsible loess, and the loess in the unirrigated area is highly collapsible. As the water content of the slope increases from 4% to 20%, the total slope displacement is predominantly horizontal, increasing from 12 mm to 140 mm, the lateral pressure coefficient also gradually increases. In saturated state, the maximum lateral pressure reaches 123 kPa when collapsible deformation occurs, the collapse-type lateral pressure coefficient increases of up to 1.4 times, and horizontal deformation is also correspondingly large. Collapse action leads to tensile stresses in the upper part of the slope, making it prone to collapse-type crack formation in the soil below the back edge of the landslide at depths of up to 5 m, resulting in dominant channels of surface water infiltration and forming landslide scarps. What’s more, lateral pressure pointing to the critical surface is generated inside of the slope due to the stress concentration, which expands to the interior of the slope along the collapse-type crack of the slope shoulder, and shear damage occurs along the curved weak zone until instability occurs. Considering that the promotion of landslides by lateral pressure is more consistent with the actual situation and can better characterize the influencing factors of irrigated loess landslides. The results of this research help to improve loess landslide–induced disaster theory, and provide reference for the precise early warning of loess landslide disasters.