Capacitive deionization (CDI) is an emerging technology that uniquely integrates energy storage and desalination. In this work, porous carbon nanosheets (PCNSs) with an ultrahigh specific surface area of 2853 m2/g were fabricated by the simple carbonization of starch followed by KOH activation for the electrode material of photovoltaic CDI. The CDI cell consisting of PCNSs electrodes exhibited a high salt adsorption capacity (SAC) of 15.6 mg/g at ∼1.1 V in 500 mg/L NaCl as well as high charge efficiency and low energy consumption. KOH activation played a key role in the excellent CDI performance as it not only created abundant pores on the surface of PCNSs but also made it fluffy and improved its graphitization degree, which are beneficial to the transport of ions and electrons. PCNSs are supposed to be a promising candidate for CDI electrode materials. The combination of solar cells and CDI may provide a new approach to reduce the energy cost of CDI and boost its commercial competitiveness.