[WT5HZ]Abstract: [WT5BZ]It is qualitatively described the relationship between the hardness of rock in catchment and the triggering frequency of debris flow: the harder of rock in catchment, the lower frequency of debris flow is. But there is no quantitative relationship of the hardness and the triggering frequency. In this paper, 17 catchments of debris flow in west Sichuan Province were investigated. The hardness of catchment, the triggering frequency of debris flow, and their relationship were researched. One can obtain the results from these research: ①The hardness of rock in the formation area is almost the same as the hardness of rock in the deposition area of debris flow catchment; ②The hardness of catchment calculated by geology map is almost the same as the hardness of catchment obtained by investigated in field; ③The relationship between the hardness of catchment and the triggering frequency of debris flow is: the (extremely) low frequency debris flows often happen in the hard rock area with the hardness ≥ 8, and the middle or high frequency debris flows often happen in the soft rock area with the hardness < 8. Finally, the comparing was made for the hardness of catchment and triggering frequency of 39 catchments of debris flow in other areas. It verified the boundary of (extremely) low frequency and middle or high frequency determined by hardness of 8 obtained in this study.