The classification of the object root types is the basic work of constructing the domain knowledge graph, and the popular public knowledge repository is not classified according to the characteristics of the domain data. For instance, the only root type of SUMO is entity, which brings defects to the domain knowledge expression. Knowledge such as texts, videos and pictures, and their relationships cannot be fully expressed. The object root types of domain knowledge graph ontology should include not only the entity type but also the event type, the text type and the multimedia type. Based on the four root types, types are extended to express the domain knowledge. In this way, various typical domain scenarios can be well described. The proposal has a good knowledge system and clear classification ideas in the development of the actual knowledge graph tools.