Abstract: The total amount of the loose masses due to collapses and landslides by Wenchuan Earthquake reached about 28?108m3.the loose masses can provide abundant materials for debris flows for long term.The critical rainfall has decreased significantly.The debris flow events have occurred at high frequency and in group.Their discharges have increased.Blocking of main rivers by debris flows have occurred more frequently post the earthquake and led serious secondary disasters in the quake-hit areas.In order to evaluate the probability of blocking river by debris flow,the basic data of debris flow gullies are obtained on the basis of field investigation and scale in different precipitation frequency according to a hydrological model.Then based on the above data and the differentiation formula of debris flow blocking river,this paper assesses the degree of debris flow blocking main river of the study area.It then defines the threshold value of the differentiation formula combined with debris flows that have occurred in the study area and blocked rivers.As a result,it can be used to determine the probability of debris blocking river more accurately.Seven debris flow gullies of the study area are evaluated at different precipitation frequencies.The results show that the debris flow of Niumian gully and Guanshan gully can block the Minjing river under the 50 years return period precipitation.The debris flow of Shaofang gully,Hongchun Gully and Mozi gully can block the Minjing river under the 10 years return period precipitation.The debris flow of Er gully and Taiping gully can block the Minjing river under the 5 years return period precipitation.The assessment result agrees with the phenomena of field investigation and can provide scientific supports for the prevention,elusion and mitigation of debris flow hazards and reconstruction project in the earthquake area.