Abstract: Through the field investigation of Sanyanyu Valley in Zhouqu county where occurred the giant flush flood and debris flow on August 8,2010,the author summarized the geological environment background of loose solid material sources from several aspects of topography,stratum lithology,geological structure,hydrological geology,new tectonic movement and earthquake.Investigation results show that:①The loose solid matter of Sanyanyu debris flow mainly include collapse deposit,landslide deposit,eluvial deposit,talus deposit and trench deposit.②The total volume of loose solid matter of Sanyanyu valley is about 4079.42×10⁴m³,which can convert to debris flow nearly 2644.54×10⁴m³(64.8%).③The fundamental mode of Sanyanyu Gully loose mass converted to the components of debris flow included collapse and landslide deposited directly or fluvial eroded and transported by jamming trench;The fine grain material of gully slope's remnant slope sediment,the landside mass,the collapse body flowed into the trench under the rainfall's surface erosion function,supplied the debris flow and increased its viscosity;The early debris flow deposit in trench was eroded and transported directly,constituted the debris flow mass.