The critical rainfall lines of 35 debris flow gullies are discussed inthis paper.Their distribution laws are found out by the cluster analysis.According to the rainfall data observed at the debris flow gullies andpartial meteorological observatories or hydrometric stations,the author mo-unted the rainfall intensity and effective rainfall, then draw thechart. The critical rainfall lines of debris flow gullies are distributed inladder. Its tendency has no relation to the scale or the characters ofdebris flow and it is not either closed with the basin area, the length ofthe main gully, relative difference of height and average gradient ofmain gully beds, etc. .However, it has a close relation to the gradient ofslope in the formation region of debris flows and their occurence fre-quences. Especially, it has close relation to the forms of supplying solidmaterial for debris flows.