There are a variety of situational elements, element attributes, and intricate relationships between the elements in cyberspace. The deccription of these information directly impact the accuracy, completeness and effectiveness of the cyberspace situation model. The knowledge representation is adopted to describe the key information elements in cyberspace. Firstly, the characteristics of cyberspace situation information knowledge are analyzed, and the important role of knowledge representation in cyberspace situation information is proposed. Secondly, the ontology?based knowledge representation theory is studied, and the motivation of adopting ontology to represent the cyberspace situation is analyzed. Finally, an ontology?based knowledge representation method for cyberspace situational information elements is proposed, and the realization method of that is introduced in detail. The research can effectively promote the perception, modeling and visualization of cyberspace situation, and provide an effective reference for the research of cyberspace related technologies.