Constructing large-scale ontology in clinic domain needs collaboration of multidisciplinary experts. Problems of lacking coherence among experts hinder the wide use of ontology in sharing the electronic medical records generated by different hospital information systems. In this paper, an ontology construction method is proposed from the viewpoints of both topdown and bottom-up. Ontology segments are firstly extracted bottom-up from the database schema of operational clinical information systems. Then, experts from different disciplines work together to merge the ontology segments top-down, and to build a global ontology system. Finally, this approach is demonstrated in a clinic data analysis project. The result shows that the method is efficient in ontology engineering.