Potassium-ion batteries (KIBs) are regarded as potentially promising large-scale energy storage (EES) systems due to the close redox potential to lithium, the rich reserve and low cost of potassium resources. Nevertheless, the larger ionic radius of K-ion makes cathode materials for KIBs be a difficult issue. Herein, we construct Mn-based layered oxide (K0.32MnO2) microsphere, self-assembled by ultrathin nanosheets, as potential cathode electrode for nonaqueous KIBs, which delivers a reversible discharge capacity of 95.1 mAh·g−1 at 10 mA g−1. It is demonstrated that K0.32MnO2 undergoes a simple solid solution reaction mechanism for K-ions deintercalation/intercalation during the initial charge-discharge process, which is not like complex multiple processes accompanied by phase transition for Mn-based layered oxide particles. Ex situ measurements reveal that K0.32MnO2 nanosheets can provide outstanding morphology and structure stability, as well as fast electrochemical kinetics upon cycling, thus high rate capacity, improved cycling lifetime and voltage stability at low and high rates can be obtained. We also verify the practical feasibility of K-ion full cell with commercial super P carbon black anode. This novel work illuminates the electrochemical mechanism for K-ions storage in depth, which may push the study on layered oxide cathodes and accelerate the development of KIBs.