The deformation mechanism and characteristics of outwash deposit body under rainfall condition are different from those of rainfall-induced landslides. The study area is featured with characteristics of dry and warm valley climate, with small rainfall amount but large evaporation. Through water-soil chemical effect tests, mineral composition detection, scanning electron microscopy (SEM), rainfall simulation and medium scale in-situ shear test, the structural damage mechanism of outwash deposit in water is examined, and the characteristics and amplitude of strength attenuation are also investigated. On this basis, the infiltration and saturation characteristics of groundwater and the change trend of pore water pressure are analyzed by using SEEP/W module. The deformation mechanism and process of outwash deposit body are concluded in summary in line with geoenvironmental conditions. Research results unveil that when outwash deposit contacts with water, the dissolution of mineral (mainly gypsum) together with the rearrangement of mineral particles weakens the soil structure, which further develops and mitigates the interactions among large particles, and finally results in the dramatic attenuation of overall soil strength under the lubrication and softening effect of water: the value of cohesion declines by 20.6%, and internal friction angle by 34.1%. Moreover, due to the special geological condition of the study area, the effect of rainfall infiltration in gentle slope is better than that in steep slope; and the differences in saturation time, range and depth brings about the variation of soil strength. Besides, favorable free surface gives rise to creep deformation in the direction of the free surface in the front edge and tensile cracks in the back edge of deformed body, providing passage for rainfall infiltration. Cumulative creep deformation diminishes the locked segment in the front edge of deformed body, and finally induces instability.