This paper proposes a strategy of query optimization based on domain ontology. Three kinds of query optimization respectively based on is-a relation, part-of relation, and equivalent-class relation in the domain ontology are investigated. By using this strategy, a domain-specific information searching system building upon GoogleAPI and domain ontology is built, where optimized user query is processed. Comparisons of searching results of optimized and non-optimized query show the effectiveness of the strategy in improving precision and recall of information searching.