Capacitive deionization (CDI) has become a promising technology for water desalination due to its remarkable advantages including low operation cost, no secondary pollution and high rate of ion recovery. However, the majority of commercial CDI electrode materials are carbonaceous materials such as activated carbon with limited capacitance and high charge transfer resistance, which significantly hinders the wide application of CDI. Herein, we demonstrate a N-doped carbonaceous CDI electrode with a maximum ion electrosorption capacity of 19.9 mg/g, a low charge transfer resistance (1.17 Ω) and a robust regeneration performance (2800 min for 28 circles). The N-doped carbonaceous CDI electrode is the commercial activated carbon fiber (ACF) decorated with polyaniline (PANI) (ACF/PANI) electrode fabricated by in-situ electrochemical polymerization. The ACF/PANI electrode was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The pseudocapacitance of ACF/PANI electrode significantly contributed to the effectively improved CDI performance that 90.0% of sodium storage was attributed to the capacitive process and the unique porous structure of ACF/PANI electrode contributed to the other 10.0% diffusion-controlled capacity.