A new electrochemical model has been established to explain the carbonation behavior of marine sand mortars. To conduct the mathematical derivation of the new model, a curvilinear equation in Nyquist plot was acquired. The reliability of the model has been proved by way of comparing the new model with the previous. The experimental results demonstrate that electrochemical impedance behavior of mortars has been moderated in the process of the carbonation. The carbonation process can be quantitatively analyzed by the parameters fitted from the electrochemical model. Moreover, prediction of the carbonation depth can be achieved.