Loess landslide incidents have resulted in significant economic losses and human casualties in Northwest China. To investigate the triggering mechanism of such loess landslides in the south Jingyang platform, Shaanxi province, an area which is subject to loess landslides, we performed a series of field measurements for loess landslide LD37 and also undertook laboratory. Comparisons between the field measurements of LD37 and the results of the constant shear drained (CSD) triaxial test indicate that the seepage from irrigation-water infiltration triggered the loess landslides in the study area where the initiation of the local sliding surfaces was linked to gentle stratum erosion and their subsequent development was because of excessive stratum erosion. The development of slow–rapid strain cycles in the ε–p′ plot from the CSD triaxial test results indicates that loess landslides are a feature of progressive failure of the loess slope. Preventive measures to reduce irrigation-water infiltration into loess slopes should be taken to prevent similar incidents from recurring in the future.