Hard carbon (HC) is the most promising anode material for sodium‐ion batteries (SIBs). The main obstacle to HC's application is its incompatibility with the phosphate flame‐retardant additive, a common SIB additive. This incompatibility arises from the unstable solid electrolyte interphase (SEI) caused by phosphate molecule decomposition. For the first time, in this work, a new type of highly adaptive flame‐retardant electrolyte is proposed. The electrolyte ensures the stability of the electrode/electrolyte interface by the introduction of pioneering long‐chain nitrile as the SEI‐forming additive and solvated structural regulator. Long‐chain nitriles decompose before phosphate and form a stable SEI containing N element, which prevents the phosphate from inserting into the HC's layers and reduces its sodium‐storage capacity. As a proof of concept, the HC anode demonstrates a higher initial Coulombic efficiency of 80.78% in the as‐designed highly adaptive flame‐retardant electrolyte, which is about 1.8 times than that before regulation. It can cycle stably for 1250 cycles at 3 C with a capacity retention of 60%. Moreover, the electrolyte has good flame retardancy and can extinguish naturally within 1 s. In this work, an innovative method is provided for developing high‐safety SIBs based on HC anodes.