Sodium-ion batteries (SIBs) are promising candidates for large-scale electric energy storage with abundant sodium resources. However, their development is challenged by the availability of satisfactory cathode materials with stable framework to accommodate the transportation of large-sized Na+ (1.02 Å), whose continuous insertion/extraction can easily cause irreversible volumetric deformation in the crystalline material, leading to inevitable structural failure and capacity fading. Here, different from the previous synthesis efforts targeting at Na+ containing compounds, we unveil the possibility of achieving a highly reversible sodiation/desodiation process by resorting to a K+-based layered metal oxide formulated as K0.5Mn0.7Fe0.2Ti0.1O2 (KMFT), which is a P2 type in structure with a wide interlayer spacing to sit K+ (1.38 Å). We demonstrate that an initial K+/Na+ exchange can introduce Na+ into the lattice while a small amount of K+ remains inside, which plays a significant role in ensuring enlarged channels for a fast and stable Na+ diffusion. The KMFT electrode delivers a high initial discharge capacity of 147.1 mA h g-1 at 10 mA g-1 and outstanding long cycling stability with capacity retention of 71.5% after 1000 cycles at 500 mA g-1. These results provide a new design strategy for the development of stable SIBs cathodes to facilitate their future applications.