Prussian blue analogues are promising cathode material candidates for aqueous rechargeable metal-ion batteries. Although great efforts have been made on developing materials, there are still rare reports on optimizing cell performance from mechanistic understanding and studies. Here we unveil the alkali-metal-ion intercalation mechanism in Berlin green with a home-built spectroelectrochemical cell for operando X-ray Diffraction (XRD) and Raman spectroscopies, which allows us to obtain the correlated local structure, crystal structure, redox activity, and potential profiles during the charging and discharging processes. We found that the intercalation of Na+ follows a solid solution mechanism leading to a high capacity, and the intercalation of K+ follows a two-phase transition mechanism showing a high voltage. With this understanding, we propose a new strategy using a Na+/K+ hybrid cation electrolyte to realize both high voltage and energy density. This study offers a unique insight for improving the cell performance from the understanding of the reaction mechanism.