Potassium vanadium fluorophosphate (KVPO₄F) is one of the most promising cathode candidates for K-ion batteries because of its high specific capacity, voltage, and energy density. However, reducing its capacity fade remains an important challenge. This work leverages structure and electrochemical analysis to understand the capacity degradation mechanism of the KVPO₄F cathode. Interestingly, no structural degradation of the KVPO₄F cathode is detected after 200 cycles in the wide voltage window of 5.0–2.5 V (vs K/K⁺). Instead, the capacity degradation is attributed to electrolyte decomposition at high voltage (>4.5 V vs K/K⁺), which causes drying of the electrolyte and the formation of insulating layers on the cathode surface, significantly increasing the polarization. The properties of four KPF₆- and carbonate-based K electrolytes are compared, and 0.7 M KPF₆ in ethylene carbonate/propylene carbonate exhibits the highest oxidation stability and results in the best cycling stability for the KVPO₄ cathode. These findings suggest that the key to improving the cycling stability of KVPO₄F is to develop novel K electrolytes with even higher oxidation stability.