Abstract: In order to describe uncertain knowledge in the objective world, including fuzziness and randomness, uncertain extension of OWL2 syntax was carried out by introducing cloud model, and an uncertain ontology language C-OWL2 was proposed. The method was proposed without changing the existing structure of OWL2, syntax extension was carried out by using the annotation property of OWL2, and the corresponding C-OWL2 description of concepts, roles, etc. was given. Finally, the specific methods of application were illustrated with examples. In this method, OWL2 ontology was divided into two parts, precise ontology and uncertain ontology. In addition, when ignoring the uncertain part, the expression and reasoning of precise knowledge is not affected. This method is an extension of the function of fuzzy description logic, and overcomes OWL2 language's limitation of dealing with the precise ontology only.