In this paper, we propose the Semantic Information Model for Digital Libraries - DLOnto based on the theory of the Semantic Web, and give the formalized definition of and relative Ontology operations on this model, which is constructed on the basis of the Chinese Classification and Thesaurus for Libraries extended using WordNet. We use DLOnto to implement query expansions and give an algorithm for measuring semantic rank. Experiment results show that DLOnto greatly improves the semantic relativity of query results.