Abstract: The risk assessment of the debris flow liability to occurrence is a complex and uncertain problem as it is influenced by some natural conditions. In order to evaluate the liability to occurrence of the debris flow on Karakoram highway, the coupling of the interval number theory and a set pair analysis method were comprehensively applied to establishing the assessment model for the debris flow liability to occurrence based on the connectional expectation. Based on in-situ investigations and the results of the remote sensing image interpretation, 14 evaluation indexes were selected in this model. Then the values of evaluation indexes and the classification standards of each evaluation degree were also determined by the interval number theory. The connectional expectation was calculated by a set pair analysis and then combined with the weight coefficients of the evaluation indexes, the comprehensive connectional numbers of each evaluation index to the degree of debris flow liability to occurrence were introduced. Finally, the degree of the debris flow liability to occurrence was determined according to the principle of the maximum connectional number. Compared with other methods, the calculated results of an experimental example of the debris flow liability to occurrence of Karakoram highway indicate that the model can efficiently predict the debris flow liability to occurrence and simplify the analytical process of the interval number relationships, which has given a new approach to solving these similar uncertain problems in the future.