Abstract: The risk evaluation of debris flow on reservoir bank is a very complicated problem because the influence factors are random, nonlinear and uncertainty. The Wudongde reservoir area is located at the lower reaches of the Jinsha River. It is selected as a case study. The ten impact factors covering history of outbreak, geological condition, terrain condition and hazard inducing factors, are chosen according to the geological and environmental conditions of reservoir area. Furthermore, the debris flow risk classification is established and divided into four grades of extremely severe, severe, moderate and mild. The set pair analysis theory is used to analyze the impact factors represented by the interval number. A new model of connectional expectation for debris flow risk evaluation is introduced. This model can be used to analyze variation trend of debris flow impact factors. The case study shows that the new method is reliable and can simplify the analysis process of the relationship between interval numbers.