Abstract: MnO₂ has been used as a capacitor material owing to its high theoretical specific capacitance, however, its poor conductivity and cycle stability limits its electrochemical applications. The annealed ultrathin MnO_x-modified porous carbon nanofiber (APCNF-MnO_x) was successfully fabricated by self-incubation of electrospun porous carbon nanofibers (PCNF) in KMnO₄ solution and followed by heat treatment. The effects of porosity of the carbon fiber on the morphology of MnO_x and the electrochemical properties were comparatively studied by contact angle test, SEM, TEM and electrochemical characterization. The electrochemical results demonstrate that the APCNF-MnO_x electrode can keep high capacitance retention, with the specific capacity 4 times higher (20 mV/s) than that of the annealed MnO_x-modified nonporous carbon nanofiber (ACNF-MnO_x). The APCNF-MnO_x, as a cathode, is applied successfully for capacitive desalination. The desalination amount of the capacitor reaches 9.23 mg/g, which is 29% higher than that of ACNF-MnO_x.