Polyanion-type cathode materials have the potential to provide high energy density and long cycling for next-generation potassium ion batteries (PIBs) due to their polyanionic inductive effect and structural stability. However, uncontrolled solid-state synthesis of these materials can lead to native impurity defects, resulting in degradation in the high-voltage operation and capacity drop upon cycling. Here, a carbothermal reduction approach and a stable electrode/electrolyte interface construction regulated by voltage are combined to ensure ultra-long cycling PIBs with the prepared pure-phase KVOPO materials. Such a desirable material features a stable 3D crystal framework and numerous K sites, facilitating efficient and sustained K diffusion during cycling. Therefore, the high reversibility of K ions storage enables a decent discharge capacity of ∼63 mAhg after 1000 cycles at C/2 and a low-capacity decay of about 0.013% per cycle. Structural characterization and theory calculation demonstrate the exceptional structure and robust electrode/electrolyte interface of KVOPO, which successfully explains its cycling stability. Furthermore, the full cell using the commercial hard carbon delivers a specific capacity of 60 mAhg at C/2 (based on the active mass of the cathode) after 700 cycles, thus accelerating the practical applications of KVOPO materials.