The reason of giant debris flow triggering was obtained by field investigation and analysis on the debris flow formation conditions: heavy rainfall happened in the upstream of Sanyanyu Gully and Luojiayu Gully. The tendency of debris flow development was obtained by a preliminary analysis of the initiation conditions of debris flows:1) the debris flows will be triggered if there is a heavy rainfall in the upstream of debris flows gullies recently; 2) the debris flows will be triggered if there is a heavy rainfall in the upstream of debris flows gullies in a few years; and 3) the scale of debris flows 20 years later will be the same as the scale of debris flows before Wenchuan earthquake if there is no more earthquake effect this area and an event of large scale debris flow has happened in these gullies.