Abstract: After many sunny days and without much surface water,a debris flow happened in Yangweigou.It was so different from those common cases in that debris flow is caused by rainstorm,snow melt water or dam burst.Through visiting the witness for the happening process of the disaster and investigating the geo-environmental conditions,a conclusion was drawn that this debris flow is a special landslide,belonging to mud flow.Thus,research was done on the activity mechanism.