Abstract: On 26 June 2023, a catastrophic debris flow occurred in the Banzi Catchment in Wenchuan county of Sichuan province. This debris flow event is a viscous debris flow triggered by short duration heavy rainfall. The volume of debris which transported out of the catchment by debris flow is more than 8.3×10⁵ m³, and the debris flow blocked the Minjiang River. The river blockage formed a barrier lake with an area of about 0.4 km². To reveal the formation mechanism of the occurrence of the extremely large debris flow events 15 years after the Wenchuan earthquake, This paper adopts a comprehensive method of field investigation, image interpretation, topographic superposition and calculation of forced vortex. This paper also analyzes the formation mechanism of debris flow and the characteristics of river blocking. Based on the results, this paper gives some suggestions of investigation and prevention of debris flow in the Banzi catchment. The research results show that (1) the triggering rainfall is characterized by short duration and high intensity. The main initiation positions are in the channel. The main debris supply is from the debris deposition along the channel. (2) The peak discharge of debris flow is as high as 755.5 m³/s, which is the main cause of river blockage. (3) Since the Wenchuan earthquake, the debris flow activity of the Banzi Catchment has the characteristics of lag and long-term because of the coupled transport of debris on the slope and along the channel. (4) The volume of the debris source which can be eroded along the channel should be paid more attention in subsequent investigation and prevention. The research results are helpful in better understanding of the activity evolution of the post-seismic debris flow. They can also provide scientific references for the prevention of the post-seismic debris flow..