Due to the minimal electrochemical oxidation–reduction potential, the potassium (K) metal anode has emerged as a focal in K-ion batteries. However, the reactivity of the K metal anode leads to significant side reactions, particularly gas evolution. Mitigating gas generation from K metal anodes has been a persistent challenge in the field. In this study, we propose a dual protective layer design through pretreatment of the K metal anode, employing a Bi2O3 modification layer alongside a stable solid electrolyte interface (SEI) formed during the initial charge–discharge cycle, which significantly suppresses gas evolution. Furthermore, we observe that the Bi2O3 modification layer enhances K nucleation due to its strong potassiophilicity when incorporated into the substrate material. The resultant SEI, consisting of dual inorganic layers of Bi–F and K–F formed through the Bi2O3 modification, effectively mitigates side reactions and gas generation while inhibiting dendrite growth. Utilizing a Cu@BO@K host, we achieve a nucleation overpotential as low as 40 mV, with a stability of 1900 h in a Cu@BO@K‖Cu@BO@K cell and a high average Coulombic efficiency of 99.2% in a Cu@BO@K‖Cu cell at 0.5 mA cm−2 / 0.5 mA h cm−2 . Additionally, Cu@BO@K‖PTCDA also presents a high reversible capacity of 114 mA g−1 at 100 mA g−1 after 200 cycles. We believe that this work presents a viable pathway for mitigating side reactions in K metal anodes.