Reverse electrodialysis (RED) is a promising technology that directly converts salinity gradient energy into electrical energy through the directional permeation of ions across the ion exchange membranes (IEMs). Fundamental understanding of the multi-physical RED process requires a reliable description of all the related phenomena involved in the process. In this work, a two-dimensional RED model based on the Nernst-Planck framework was developed. The fluid dynamics and ion transport were modelled in a full-length cell pair domain by employing the continuity, Navier-Stokes and Nernst-Planck equations complemented by the Donnan exclusion theory and local electroneutrality. The experimentally inaccessible IEM diffusion coefficients were analytically determined using the counterion condensation theory incorporating the tortuosity effect. A numerical simulation was carried out using the developed model. The solution velocity, ion concentration and electric fields were obtained and the characteristics of ion transport were analyzed. The effects of solution inlet velocity, IEM fixed charge concentration and cell pair length on RED performance were investigated and the mechanisms governing the variations of performance parameters were revealed based on ion transport. The present model contributes to a clearer understanding of the connection between the ion transport behavior and the stack performance.