We propose a new tree similarit measure based on the Connectivity-lntegrality Principle (CI). C! is a concept from graph theory that makes the tree similarity to be measured by taking account of both the partial and the integral structures of trees. We have proved in theory that the new measure has better flexibility than the other lavo typical similarity measures when given a variety of common substructures benveen two trees. We apply this new measure to a specific-domain QA system to undertake the task of sentencelevel disambiguation. Experimental results show that, the new model can effectively enhance the rejection rate that aims at irrelevant documents