Antecedent rainfall plays an important role in the triggering mechanism of debris flow. The glacial debris flow triggering experiments of different original soil water content were carried out to simulate the washed glacial drift triggering process under varieties of antecedent rainfall. The results showed that：(1) The evolution type of glacial debris flow in study area preliminarily could be defined as slope-slide model. (2) Seepage played a leading role at the low original soil moisture content. A large number of fine particles were occurred, leading to the intergranular porosities increased to the original two times when grain-contact stress falling; then the rearrangement of the soil structure and enlargement of the pore-water pressure lead to the soil particles failure. It could be shown in the pore-water pressure as a “fall-uprush-fall” form. (3)Both Seepage and scour were key factors at higher original moisture content. The starting process of debris flow was intermittent, and the triggering moment that was the peak moment of pore-water pressure. (4) Both pore-water pressure and the volumetric soil water content showed periodical curves. The original soil water content was negatively associated with the debris flow starting time. (5) Based on the experiment phenomenon and the characteristics of curves, debris flow could be divided into four steps in this study. And the mechanism could be concluded to seepage erosion, soil front end framework and runoff scouring.