Layered K0.67-xNaxNi0.17Co0.17Mn0.66O2 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) cathodes for potassium-ion batteries were successfully prepared by a co-precipitation method. The influences of Na substitution were studied in detail and it was found that the Na content could change the crystalline structure (from single phase to biphase) and eventually affect the electrochemical properties. The moderate amount of Na could stabilize the layered structure, activate the redox reaction of transition metal ions and further improve the cycling and rate performance. The P3/P2-type K0.37Na0.3Ni0.17Co0.17Mn0.66O2 composite displayed enhanced cyclability with a reversible capacity of 86.1 mAh g−1 and 78.8 mAh g−1 kept after 100 cycles at 20 mA g−1, which was better than the pristine P3-K0.67Ni0.17Co0.17Mn0.66O2 (76.1 mAh g−1 and 65.3 mAh g−1 at the same condition, respectively). In addition, the K0.37Na0.3Ni0.17Co0.17Mn0.66O2 sample delivered a high capacity of 62 mAh g−1 even at 100 mA g−1, much higher than that of K0.67Ni0.17Co0.17Mn0.66O2 (47.8 mAh g−1). These results proved that appropriate metal doping could be a feasible solution to improve the performance of layered ternary cathode for potassium-ion batteries.