Development of sodium and potassium ion batteries with greater energy density is gaining great attention. Although recently proposed alloying anodes (e.g., Sn and Bi) demonstrate much higher capacity than classic carbon anodes, their severe capacity fading hinders their practical applications. The failure mechanism has traditionally been attributed to the large volumetric change and/or their fragile solid electrolyte interphase (SEI). However, herein we present a completely new concept and approach based on electrolyte engineering to stabilize alloying anodes. This approach results in unprecedented high capacity (>650 mAh g–1) and stability (>500 cycles) of alloying anodes by simply tuning the electrolyte compositions, without the need for nanostructural control and/or carbon modification. We confirm that the cation solvation structure, particularly the type and location of the anions in the electrolyte, plays a critical role in alloying anode stabilization. We further present a new anionic and alloying anode reaction model showing that the root cause of the capacity fading in these alloys is dictated by the properties of the anions and not only the volume change or fragile SEI effect. Our model elucidates the failure mechanism in alloying anodes and provides a new guideline for electrolyte design that stabilizes alloying anodes in emerging mobile ion batteries.