The semantic web is an extension of World Wide Web and kind of intelligent network, and it can understand human language. As foundation of the semantic web, Ontology is composed by the finite terms and their relationship. Natural language is the most direct method for knowledge representation. Natural language understanding provides a new way to the research for expert system knowledge acquisition. This paper first analyzes the structure and function of the natural language processing system and proposes construction of natural language processing system based on Semantic Web ontology. The experiments prove that the natural language processing system based on ontology is more efficient.