The Yeniu gully in Wenchuan country outbreaked a debris flow on July 3,2011.It blocked the 2/3 of channel Minjiang River, causing huge economic losses.This paper analyzed the formation conditions, disaster characteristic and the formation mechanism according to the field investigation.This paper also studied the possibility of blocking the river and the trend of development. The results show that the Yeniu gully changed into a debris flow after the 5.12 Wenchuan earthquake.The characteristics of it are persistence, paroxysm and group occurrence. The initiated process can be summarized "fire-pipe effect". The overland flow of water concentrated into steep channels runoff and sources from gully-bed loose deposits. The forming process can be divided into three stages: shock starting phase, acceleration crevasse phase and accumulation phase. It can cause river blocking when the debris flow is excited by P=1% and above.