A great number of existing XML documents in various domain such as Electrical Business have to be maintained in order to constantly adapt to a dynamically changing environment to keep pace with business needs. A DTD or XML Schema in its current textual form commonly lacks clarity and readability, which makes the maintenance process tedious and error-prone. This paper presents an approach to reverse engineering the XML documents to conceptual model, which makes the XML documents more close to real world and business needs, let the designers quickly gain a picture of the overall structure of XML documents in order to improve its quality, increase the maintainability and reusability. In this paper, the conceptual model is described by UML class diagram, a three-level model is defined, and a novel approach for extracting various structure and semantic information from existing DTD is given, especially the inheritance structure can be inferred from the DTD structure.