Engineering the structure and chemistry of solid electrolyte interface (SEI) on electrode materials is crucial for rechargeable batteries. Using hard carbon (HC) as a platform material, a correlation between Na+ storage performance, and the properties of SEI is comprehensively explored. It is found that a “good” SEI layer on HC may not be directly associated with certain kinds of SEI components, such as NaF and Na2O. Whereas, arranging nano SEI components with refined structures constructs the foundation of “good” SEI that enables fast Na+ storage and interface stability of HC in Na‐ion batteries. A layer‐by‐layer SEI on HC with inorganic‐rich inner layer and tolerant organic‐rich outer flexible layer can facilitate excellent rate and cycling life. Besides, SEI layer as the gate for Na+ from electrolyte to HC electrode can modulate interfacial crystallographic structures of HC with pillar‐solvent that function as “pseudo‐SEI” for fast and stable Na+ storage in optimal 1 m NaPF6‐TEGDME electrolytes. Such a layer‐by‐layer SEI combined with a “pseudo‐SEI” layer for HC enables an outstanding rate of 192 mAh g−1 at 2 C and stable cycling over 1100 cycles at 0.5 C. This study provides valuable guidance to improve the electrochemical performance of electrode materials through regulation of SEI in optimal electrolytes. Building a high‐quality solid electrolyte interface (SEI) layer on electrode material is the key to the high performance of rechargeable batteries. A correlation between the SEI quality, that is, structure and chemistry, and the Na+ storage kinetics and mechanism in hard carbon material is demonstrated in Na‐ion batteries, which effectively guides improvement of battery performance through the engineering of the SEI layer.