Nitrogen-enriched hierarchical porous carbon nanofibers (HPCNFs) with superior capacitance and outstanding mechanical strength are facilely fabricated by electrospinning of Polyacrylonitrile/Magnesium acetate (PAN/Mg (OAc)(2)) precursor, preoxidation and carbonization of composite nanofibers, which use PAN as carbon source and Mg(OAc)(2) as precursor of MgO template. The introduction of Mg(OAc)(2) not only forms the hierarchical porosity with a large specific surface area, but also improve the thermoplasticity and mechanically flexibility of HPCNFs. Under two-electrode configuration, the as-prepared HPCNFs exhibit a high specific capacitance of 263 F g(-1) at a current density of 1.0 A g(-1), good rate capability with a capacitance retention of 157 F g(-1) up to the current density of 100 A g(-1) and excellent cycling stability of 94.2% over 10,000 cycles in 6 M KOH electrolyte owing to the synergistic effect of the hierarchical porosity and high nitrogen-enriched level of HPCNFs. Moreover, the flexibility test of HPCNFs demonstrates the excellent mechanical flexibility of HPCNFs, indicating their promising application in flexible energy storage devices.