Multi-ontologies environments are common in semantic web and knowledge representation applications. The key problem of these applications is how to represent the complex relationships between ontologies. For the purpose of solving the problem, this paper analyzed and classified the relationships between multi-ontologies, then proposed the theory of bridge ontology. The bridge ontology is a peculiar ontology, has four-layer structures, and is able to express the twelve kinds of relations between multi-ontologies. Subsequently, on the premise of keeping the semantic consistency, the detailed discussions focused on the semantic checking and refining methods for the multi-ontologies with bridge ontology. Bridge ontology can be created and maintained conveniently, and is effective in the applications based on multi-ontologies. It has the advantages of low-cost, scalable, robust in the web circumstance, avoiding the unnecessary ontology extending and integration, and promoting ontology reuse.