From the view of distributed application, it has the two important points to build ontology: sharing domain knowledge, reuse of domain knowledge. There are some problems in the present distributed applications based on ontology: It may use different terms for a same concept in different systems and it does not facilitate the information interchange between heterogeneous systems. Also, it lacks of a rigorous formal logic description for verification. Considering the application in the Internet, in this paper, we present a uniform expression format based on XML/RDF to describe the ontology, and extend PSL by creating new PSL Extensions to define the ontology of location, organization, resources, activity, process and product. These provide the mobile agent application system formal description language that supports the verification of the system and the translation with other application systems.