In order to study the wetting failure mode and mechanism of widening subgrade in selfcollapsible loess areas, the rain system and deformation field test system were installed in geotechnical centrifuge of Chang’an University. The centrifugal modal test consistent with the actual stress was conducted using the undisturbed collapsible loess. The settlement law of widening roadbed flooded by rainfall was studied and the damage mode of widening roadbed was analyzed according to the test results. The test results show that the foundation under widening roadbed producea a large settlement under the action of gravity load whose largest value appears in widening roadbed centroid place, resulting in a poor coordination deformation between new and old roadbed. The local instability of the foundation under widening roadbed caused by continuous rainfall poses a great threat to the safety of the upper widening roadbed. The formation of widening roadbed is a gradual process: First is the emergence of the collapsible caves. Next, appear the cracks in toe. Fanally comes the large deformation caused by the wetting toes which gives rise to the damage of pavement even landslide.