Lithium metal anodes (LMAs) are ideal anode candidates for achieving next-generation high-energy-density battery systems due to their high theoretical capacity (3680 mA h g-1) and low working potential (-3.04 V versus the standard hydrogen electrode). However, the non-ideal solid electrolyte interface (SEI) derived from electrolyte/electrode interfacial reactions plays a vital role in the lithium deposition/stripping process and battery cycling performance. The composition and morphology of a SEI, which is sensitive to the outside environment, make it difficult to characterize and understand. With the development of characterization techniques, the mechanism, composition, and structure of a SEI can be better understood. In this review, the mechanism formation, the structure model evolution, and the composition of a SEI are briefly presented. Moreover, the development of in situ characterization techniques in recent years is introduced to better understand a SEI followed by the properties of the SEI, which are beneficial to the battery performance. Furthermore, recent optimization strategies of the SEI including the improvement of intrinsic SEIs and construction of artificial SEIs are summarized. Finally, the current challenges and future perspectives of SEI research are summarized.