Abstract: Developing new energy, reducing fossil energy consumption, and building a green and low-carbon energy system are important strategies to achieve carbon neutrality. To realize the grid connection of new energy generation, rechargeable potassium-ion batteries (KIBs) are expected to be used in large-scale energy storage, considering sufficient potassium resources and potential high-energy density. Anode, an essential battery component, directly determines the battery safety, cycle life, and energy density. Among various anodes, alloys can provide high theoretical specific capacity based on the multi-electron reaction mechanism, which is promising in terms of improving the energy density of a full battery. Their K-storage voltages also stay away from the deposition/stripping potentials of metallic K, thereby enhancing battery safety. However, the dramatic volume variation upon alloying and dealloying leads to electrode pulverization and capacity fading in traditional carbonate-based electrolytes. An effective method for stabilizing an alloy-based anode structure is the construction of a stable electrode–electrolyte interface by electrolyte optimization, which has the advantages of a simple process and low cost. Accordingly, the utilization of interfacial engineering to achieve stable alloy anodes has been frequently reported in the past few years. This topic includes the following: (1) regulation of the components of solid electrolyte interphase (SEI) layers to improve mechanical strength and ionic conductivity, buffer volume fluctuation, and reduce electrode corrosion; (2) adjustment of the solvated structure of K⁺ to enhance the diffusion rate and inhibit the electrolyte decomposition; and (3) utilization of solvent molecular chemisorption on the electrode to induce its microstructure change, improve the electrolyte wetting ability, and relieve volume change. The SEI is a passivation film generated on the electrode surface at the initial battery operation stage. Research on its structure, component, and formation mechanism is still basic due to its instability and complexity and limited research methods. Whether the solvated structure of cation and electrolyte adsorption affect the SEI structure and composition remains unclear. How the solvated structure and the electrolyte adsorption improve the electrode stability must also be further studied. This review covers recent research progress on the interfacial interaction between alloy anodes and electrolytes in KIBs, summarizes the electrolyte optimization strategies, analyzes the potassium storage mechanisms and electrochemical performance of alloy anodes, and highlights the interfacial interaction mechanisms. More importantly, this paper provides new insights for the future development of KIB electrolytes.