The integration of Building Information Model (BIM) with Geographic Information System (GIS), not only can make elaborate 3D models great reuse, but also can realize the comprehensive digital expression from geometry to physical and functional properties. The integration has become one of the main means to three-dimensional modeling of the new generation Digital City. This paper selected the typical BIM model data format (RVT), proposing a semantics-constrained conversion approach from RVT model to CityGML model. With the rich semantic information of BIM model as constraint, this paper achieved geometric simplification and transformation of BIM model, and realized the semantic output and one to one mapping of geometry components and semantic information. At last, it is validated by selecting building models, bridge models and Heating\Ventilation\Air Conditioning (HVAC) as examples.