Highly concentrated electrolytes (HCEs) are promising for the construction of advanced lithium metal secondary batteries through inhibiting lithium dendrites in kinetics and lithium protection with an anion-derived solid electrolyte interphase (SEI). The mixed-solvent strategy is effective for obtaining electrolytes with the required properties, and the tuning of their related proportions has great impact on the solvation structures and interface properties. In this work, to break through the limitation of the lithium salt solubility in fluoroethylene carbonate (FEC) and build the anion-derived SEI, acetonitrile (AN) was employed as an occupying agent to further decrease the mole ratio of FEC to anions. Thus, FEC–AN mixed solvent-based hybrid HCEs were systematically investigated. The reduced viscosity, improved ionic conductivity and enhanced oxidation stability of HCEs were obtained. At the optimal proportion, the expected SEI was achieved, ensuring high coulombic efficiency and long cycle stability of the lithium metal anode. This work shows the necessity of the mixed solvent regulation strategy for HCE improvement.