With the rapid development of artificial intelligence technology, the contradiction between information explosion and knowledge shortage has become increasingly prominent. Knowledge extraction technology is an effective way to resolve the above contradiction. It automatically, quickly, and accurately extracts massive, heterogeneous data of knowledge that interests people and stores it in structured knowledge. This paper first introduces the research significance of knowledge extraction technology and its vital role in the construction of knowledge graphs. Then we focus on the key technologies of knowledge extraction including named entity recognition, relation extraction, and attribute extraction. We also take technology services as an example to introduce the specific application of knowledge extraction and knowledge graph. Finally, the problems that need further attention in the field of knowledge extraction are put forward.