In order to explore the rainfall threshold of wide moderate narrow steep debris flow along the G213 line from Dujiangyan—Wenchuan. Remote sensing images before and after earthquakes and geographic information system (GIS) technology were used to establish a comprehensive terrain factor critical model, and topographic factors of 43 gullies in the study area were extracted. A comprehensive terrain factor critical model was established based on principal component analysis and SPSS software to analyze the correlation of various factors. By introducing rainfall factors and utilizing support vector machine classification models, rainfall threshold models for different channel shapes were obtained. The results show that when the comprehensive terrain factor is greater than 0.51, it is a narrow and steep channel. When it is less than or equal to 0.51, it is a wide and gentle channel. The model validation of some channels in Beichuan achieves an accuracy of 99%. Based on the classification of channel types based on comprehensive terrain factors, the influence coefficient between comprehensive terrain factors and rainfall factors is obtained through mathematical analysis of rainfall data in the study area. By obtaining the rainfall intensity I and the rainfall duration D, then utilize a random vector machine classification model was used to obtain I-D rainfall threshold models for wide and narrow steep channels. The I-D rainfall threshold model indicates that the larger the influence coefficient, the greater the terrain factor, which is more conducive to the occurrence of debris flows. The smaller the rainfall threshold requires to trigger debris flows, such as narrow and steep debris flows, and vice versa. The research results can provide reference for the post earthquake debris flow rainfall threshold in different regions, channel types, and time periods.