Potassium ion-based dual-graphite batteries (KDGBs) emerge as promising devices for large-scale applications due to their high voltage, low cost, and environmental friendliness. However, conventional KPF₆/carbonate-based electrolytes suffer from severe oxidation decomposition, low concentration, and flammability, which limit the capacity and cyclability of KDGBs. Herein, a nonflammable potassium bis(fluorosulfonyl)imide/triethyl phosphate (KFSI/TEP) electrolyte was designed for KDGBs. When the salt-to-solvent molar ratio increases to 1:1.3, graphite cathode operated at the cut-off potential of 5.2 V exhibits much enhanced capacity, excellent rate capability (26.4 mAh∙g⁻¹ at 1.0 A∙g⁻¹), and superior cyclability with 98% capacity retention after 350 cycles. Inorganic compounds-rich electrode/electrolyte interphase layers derived from the preferential decomposition of FSI⁻ anions ensure good compatibility of the 1:1.3 KFSI/TEP electrolyte with K metal and graphite anodes. Based on this electrolyte, as-assembled KDGBs show high operation voltage of 4.3 V and good cycling performance. This work provides feasibility for developing long-life and safe-operation DGBs.