The effects of precipitation on debris flows on slope in Beibei District,Chongqing City,were investigated after the study of the formation conditions and their relations with the geologic,geographical and climatic conditions.The data for debris flows and precipitation during 1962-2003 were analyzed with a case of debris flows studied specifically.The analyses indicate that precipitation is the critical factor which causes the occurrence of debris flows on slope in Beibei District.The occurrence of debris flows is found to have a close relation with antecedent precipitation and one hour rainfall intensity.It is followed from the analysis of indexes that a debris flow may occur in Beibei District as the amount of one hour rainfall reaches 40 mm,the rainfall index,160,and the comprehensive index,650.