Irrigation-induced loess landslides are common in the transitional slopes between loess platforms and river terraces in northwest China. A typical area of the south Jingyang platform, Shaanxi Province, was selected to investigate such phenomena. Approximately 50 landslides involving loess flowslides and loess slides have occurred since the irrigation of this platform began in 1976. Flowslides can travel a long runout distance at high speed and show a small apparent friction angle. Due to long-term irrigation, the groundwater table in the study area has been substantially raised. To clarify the failure mechanism of loess under these conditions, anisotropically consolidated undrained triaxial tests (ACU) and constant shear drained tests (CSD) were conducted on undisturbed specimens retrieved from the study area. The test results revealed that loess exhibited a sudden collapse behavior with the increase of pore-water pressure in a drained condition. The resultant loss of strength in an undrained condition was related to the development of pore-water pressure that reduced the effective stress. For a flowslide, we conclude that the groundwater rises first and causes the collapse of loess in the bottom portion of the slope, which then leads to undrained mobilization. Following the initiation, excess pore-water pressure is generated near the contact between sliding material and river terrace, leading to mobility of the flowslide.