The crystal phase structure of cathode material plays an important role in the cell performance. During cycling, the cathode material experiences immense stress due to phase transformation, resulting in capacity degradation. Here, we show phase-engineered VO₂ as an improved potassium-ion battery cathode; specifically, the amorphous VO₂ exhibits superior K storage ability, while the crystalline M phase VO₂ cannot even store K⁺ ions stably. In contrast to other crystal phases, amorphous VO₂ exhibits alleviated volume variation and improved electrochemical performance, leading to a maximum capacity of 111 mAh g⁻¹ delivered at 20 mA g⁻¹ and over 8 months of operation with good coulombic efficiency at 100 mA g⁻¹. The capacity retention reaches 80% after 8500 cycles at 500 mA g⁻¹. This work illustrates the effectiveness and superiority of phase engineering and provides meaningful insights into material optimization for rechargeable batteries.