This paper presents the findings of an investigation by the Delphi Method and the process of summarizing experts' opinions by making use of Grey systems theory. The investigation was made to fifty-three experts aimed in finding out the main factors in the formtion of torrential rain debris flow.The Grey statistics method is used to summarize the experts-' opinions. As a supplement，the Grey relational grade analysis is made on the data obtained from surveying and measuring. Based upon the conclusion from these studies, the forming of torrential rain debris flow is imputed toeleven /main factors classified into three groups according to their importance.