Abstract: The investigations on 21 debris flows on mountain slope in Beibei District, Chongqing makes clearly that gradients are very important to debris flow on mountain slope.After counting and analyzing a great deal of the drainage area of debris flow on mountain slope, we find that the distribution of gradients is the normal distribution.According to the distributive laws of the average gradients of the drainages areas of debris flow on mountain slope, we make sure that the critical gradients can be classified into three kinds.The average of the first kind of the critical gradients is 32.4 degrees.The debris flows belonging to the first kind of the critical gradients are apart from residential areas and human being’ activities are feeble, and at the same time, their vegetation rates are higher.The average of the second kind of the critical gradients is 27.3 degrees.The debris flows belonging to the second kind of the critical gradients are located in woodlands and farmlands, and human being’ activities are acute and the vegetation rates are lower .The average of the third kind of the critical gradients is 36.8 degrees.The debris flows belonging to the third kind of the critical gradients are in the Guanyin gorge.They are controlled by the terrain condition.We have quantitatively explained the relationship between the crucial gradients and the debris flow on mountain slopes.It can afford services to the dividing ecological frailty sections, engineering construction, preventing and decreasing disasters, and the government decision-making.