Abstract: This study constructed capacitive deionization(CDI) systems with different numbers of stacked units, using activated carbon as electrode material without ion exchange membranes. The effects of the number of stacked units on freshwater productivity, salt adsorption capacity and energy efficiency of the CDI system were explored. The results show that CDI systems with 1, 2, and 4 pairs of electrodes achieve the total salt removal quantity of 10.11, 23.41, and 34.22 mg at the same flow rate, respectively. Furthermore, the average concentration reduction(ΔC) and the thermodynamic energy efficiency(η_TEE) of the CDI systems increase dramatically with the increase of the electrode units: ΔC increased from 2.16 mmol/L to 7.31 mmol/L, and η_TEE increased from 0.66% to 2.17%.In the CDI system with two pairs of electrodes, the charge efficiency and the salt adsorption capacity(Q_SAC) do not be affected by the flow rate, while the ΔC, energy consumption, and η_TEE of the system all significantly decrease with the increase of the flow rate. In addition, when simultaneously increasing the stacked units and flow rate with the same factor, the total salt removal quantity of the CDI system was 10.11, 22.25 and 32.89 mg at the stacked units of 1, 2, 4, respectively. All of these three systems achieve the average desalination concentration of about 2 mmol/L. Therefore, increasing the number of stacked units can effectively expand the scale of CDI system.