Rechargeable sodium ion batteries (SIBs) are surfacing as promising candidates for applications in large-scale energy-storage systems. Prussian blue (PB) and its analogues (PBAs) have been considered as potential cathodes because of their rigid open framework and low-cost synthesis. Nevertheless, PBAs suffer from inferior rate capability and poor cycling stability resulting from the low electronic conductivity and deficiencies in the PBAs framework. Herein, to understand the vacancy-impacted sodium storage and Na-insertion reaction kinetics, we report on an in-situ synthesized PB@C composite as a high-performance SIB cathode. Perfectly shaped, nanosized PB cubes were grown directly on carbon chains, assuring fast charge transfer and Na-ion diffusion. The existence of [Fe(CN)(6)] vacancies in the PB crystal is found to greatly degrade the electrochemical activity of the Fe-LS(C) redox couple via first-principles computation. Superior reaction kinetics are demonstrated for the redox reactions of the Fe-HS(N) couple, which rely on the partial insertion of Na ions to enhance the electron conduction. The synergistic effects of the structure and morphology results in the PB@C composite achieving an unprecedented rate capability and outstanding cycling stability (77.5 mAh g(-1) at 90 C, 90 mAh g(-1) after 2000 cycles at 20 C with 90% capacity retention).