Potassium-ion batteries (KIBs) are a promising alternative to Lithium-based energy storage systems owning to the low cost and rich abundance of potassium resources, but are facing challenges in designing low-cost hosts that can reversibly accommodate large-size K⁺ with fast diffusion kinetics. Herein, we report a novel 3D inorganic open framework of Ca₂Nb₂O₇ (CNO) as an anode for KIBs. The open framework structure affords interstitial vacancies available for storing K⁺ and allows a facile diffusion of K⁺, thus resulting in excellent structural stability and fast reaction kinetics. The CNO electrode delivers a reversible specific capacity of 65.3 and 52.2 mAh/g at 5 and 10 mA/g, respectively. Moreover, CNO exhibits excellent long-term cyclability with 92.53% capacity retention over 700 cycles at 10 mA/g. This will trigger more investigations into open-framework-based materials for stable and fast KIBs.