The worldwide large-scale commercial deployment of 5G has commenced in 2020 for supporting enhanced Mobile BroadBand (eMBB),ultra-Reliable and Low-Latency Communications (uRLLC),and massive Machine-Type Communications (mMTC) services.Nevertheless,the upsurge of Artificial Intelligence (AI)-powered applications,the developmental law of one-decade-one-generation of wireless communications and the inherent limitations of 5G have also been spurting the industry and academia to dedicate their efforts to the research of future 6G wireless systems.6G will be a disruptive,pervasive,intelligent,and endogenous wireless system,which will revolutionize all walks of life and accelerate the transformation and innovation of the global society.In this paper,we present a forward-looking,comprehensive and in-depth analysis and technical identification of 6G.Specifically,we firstly introduce the fundamental theories of 6G in terms of potential requirements.Then,we focus our attention on the discussion of promising key technologies in terms of spectrum,air interface,delay,access,energy consumption,coverage,AI,electromagnetism,interaction,etc.