During the new urbanization construction and management, data integration of BIM and GIS is not only beneficial for realizing comprehensive implementation of buildings, but also obtaining the spatial analysis and indoor and outdoor integrated expression with the support of comprehensive outdoor information. What is noticeable is that the current challenge is that two different data formats, BIM and GIS, fails to directly achieve data interoperability. Therefore, the differences between IFC and CityGML in document format are analyzed initially, and the conversion of multiple simple building IFC models to CityGML3.0 LOD2 and LOD3 is implemented. Secondly, concerning the semantic loss in the above data integration experiments, the CityGML3.0 data format of GIS is extended, and a CityGML B-Con extension model is proposed, as well as a one-to-one mapping relationship from IFC to CityGML3.0 is improved. Finally, the conversion of multiple IFC models of buildings with different complexity levels to the CityGML B-Con extension model is successfully implemented. The experimental results show that the CityGML B-Con model is capable of achieving data integration between BIM and GIS. Standing from a macro perspective, the implementation qualify of the converted building appearance can be enhanced. Standing from a micro perspective, the subclasses of BuildingConstructiveElement and BuildingInstallation are distinguished, and building components, such as IfcStair, IfcRamp, and IfcBeam can be converted into CityGML through one-to-one mapping, which overcomes the demerits of the original CityGML standard in the representation of building components, effectively reducing the loss of semantic information from BIM source data.