Multi-ontologies environment is common such as the semantic annotation applications based on multi-ontologies. Based on analyzing and classifying the relationships between multi-ontologies, this paper proposes the idea of bridge ontology, and presents its detailed structure too. To deal with the semantic annotation problem based on multi-ontologies, a new approach is proposed which employing bridge ontology to express the complex relationships between multi-ontologies. The bridge ontology is a peculiar ontology, which can be created and maintained conveniently, and is effective in the applications based on multi-ontologies. The approach using bridge ontology has the advantages of low-cost, scalable, robust in the web circumstance, avoiding the unnecessary ontology extending and integration, and promoting ontology reuse.