Nonlinear diffusion (NLD) behavior for the Prussian blue (PB) electrode was revealed using an in situ method combining the advantages of the potentiostatic intermittent titration technique (PITT) and chronoabsorptometry, namely, the PITT-chronoabsorptometry method. By applying the PITT-chronoabsorptometry method, the disruptive influence of attack by dissolved O-2 was eliminated and the diffusivities were precisely determined. Prior to the diffusivity determination, the charge-transfer plane and diffusion species involved in the PB-Everitt's salt (ES) redox process were proposed based on the morphological open structures of PB and the binary-solution model. This model suggests that a partially reduced PB film could be viewed as a binary solid solution of PB and ES. The diffusivity measured for the PB-ES redox system is a function of the equilibrium potential. The potential corresponding to the minimum diffusivity is close to the peak potentials obtained from the cyclic voltammogram. This characteristic is very similar to a common diffusivity-potential dependence found in lithium-inserted electrodes and implies that such NLD behavior could be a common behavior for other insertion compounds. Moreover, this work not only provides the opportunity to further understand the PB electrochromic process but also proposes a robust in situ approach for determining diffusivity at other electrochromic electrodes.