Abstract: 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 recorded rainfall was 77.3mm in one hour from 23 to 24 oclock, August 7, 2010. The heavy rain caused powerful flash flood in the catchments,which destroyed several natural dams and prevention dams in the two gullies. Then the giant debris flows were formed. The debris flows were high density viscous debris flows. The total volume of debris flows and the volume of sediment in debris flows were estimated at 144.2104m3 and 97.7104m3, respectively. The large boulder involving in debris flows moved with powerful impact force and destroyed more than 5500 houses in depositional fans. The debris flows produced a dam with 550m in length, 70m in width, and 10m in height in Bailongjiang River. The dam blocked the river and formed a lake with 3km in length and half of Zhouqu city was inundated. As of August 18, 2010, the debris flows claimed 1287 death, with further 457 listed as missing. 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, but the scale of debris flows will be less than the scale of debris flows on August 7, 2010; （2) The debris flows will be triggered if there is a heavy rainfall in the upstream of debris flows gullies in a few years, the scale of debris flows will be a little bit less than the scale of debris flows on August 7, 2010; （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 a event of large scale debris flow has happened in these gullies.