Ueological condition,one of three debris forming factors,mainly provides the solid source for debris flow,and weathering plays very important role in the formation of solid source.The article analyzed the influence of physical and chemical weathering on the formation of solid source based on the rock firmness coefficient and concluded that:(1)The physical weathering,often caused by frost wedging in favor of the formation of debris flow,is the main reason for the development of the solid source of debris flow;(2)The chemical weathering does not provide the solid source directly while the carbanion is not in favor of the formation of debris flow.Based on the modification of the rock firmness coefficients by physical and chemical weathering,the geological factor forming debris flow is obtained,which was tested with good result in the judgment of debris flow outbreak in Wenchuan Earthquake area.