Due to the high theoretical capacity and electrode potential, Prussian blue is regarded as promising cathode material for potassium ion batteries. However, inferior structural stability, poor electronic conductivity, and ambiguous energy storage mechanism have limited the application of Prussian blue materials. Herein, a highly stable Prussian blue-polypyrrole (PB-PPY) composite has been prepared by a facile one-step method. PB-PPY displays higher discharging capacity, better rate capacity, and longer cycling lifespan than that of pure Prussian blue in potassium ion batteries. The superior electrochemical performance can be attributed to the unique synthesis strategy to reduce the content of vacancies and crystal water in Prussian blue and enhance the conductivity. Furthermore, partial K ions have been evidenced that could remain in the Prussian blue framework, which contributes the long-term cycling stability. The K ions in the framework play the role of “pillars” to support the framework of Prussian blue and relieve the structural stress during the intercalation and de-intercalation of K ions. This work will reveal a new energy storage mechanism of Prussian blue and promote the design of high stability Prussian blue in the future.