With the development of the Internet, the need of city 3D data is rising more and more, and the emphasis shifts from analysis to the sharing of city 3D data and information over the Internet. In Internet, we lack uniform 3D data standard and 3D data cannot be shared and exchanged availably. The city Geography Mark-Up Language (GML) was developed as the standard language for exchanging and sharing city 3D data in Internet. In this paper, we propose a set of data structures to obtain city 3D information from the GML file in order to extract useful information quickly, after analyzing CityGML files structure and parsing by the Document Object Model (DOM). The experiment shows that 3D data are extracted effectively, and visualized in Web application using Java, and the data structures defined in the paper are very validated.