After the industrialization of 5G cellular communications, 6G has increasingly become a research hotspot in the academia. Space-Air-Ground Integrated Network (SAGIN) is a key supporting technology for 6G because of its advantages such as high-speed transmission and expanded coverage. This paper summarizes the motivation to develop the SAGIN-assisted 6G first and introduces the current situation of SAGIN. We then try to discuss the design concept of the SAGIN-assisted 6G, and list the problem and challenges. Moreover, we propose an architecture of the SAGIN-assisted 6G and identify a series of key technologies that are needed in different layers. Finally, in order for readers to better understand our hierarchical architecture, we use a case study discussing the congestion problem in the Integrated layer.

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