One problem the present ontology faces is low construction efficiency and low reusability. For this issue, this paper proposed a new modular method for ontology based on transforming the forest into trees. In this method, the domain was decomposed into atomic subfields, each of which was modeled individually and later assembled into a modular ontology according to the inheritance relationships among the concepts and the principles for transforming a forest into trees. We examined this modular method by constructing the ontology on the undergraduate studies level, and proved that this modular method can effectively improve the reusability of the ontology, and thus was an effective approach for constructing ontology in large scale domains.