The widespread adoption of XML is creating a new set of data management requirements, such as the need to store and query XML documents. There are more and more XML documents without associated schema on the Web. It is difficult to query this kind of XML documents. This paper proposes a new XML documents storage model based on model-mapping, namely XPEV. The unique feature of model-mapping-based storage model is that no DTDs (document type declarations) or XML-schema information is required for XML data storage. Well-formed (nonDTDs) XML documents could be easily stored in relational databases through XPEV's three tables: path table, edge table and value table. Experiment shows that XPEV model excels other known model-mapping methods.