Herein we propose a low-cost, one-step synthesis of magnesium hydroxide (Mg(OH) 2) that is deposited on polyacrylonitrile (PAN) nanofibers by electrospinning. Following carbonization and etching, an in situ N-doped mesoporous carbon nanofiber (N-MCNF) network is synthesized. The synthesized flexible network is employed as a freestanding electrode for supercapacitors. The as-constructed supercapacitor based on the N-MCNFs prepared at 900 degrees C (N-MCNFs-900) can deliver excellent performance with an ultrahigh specific capacitance of 327.3 F g(-1) at a current density of 1.0 A g(-1), and remarkable cycling stability, e.g., only about 7% loss after 10 000 cycles at a constant high charging-discharging current of 20 A g(-1) in 6 M KOH aqueous electrolyte. The flexible network consisting of N-MCNFs-900 as the electrode material with long cycling stability is highly promising for next-generation high-performance supercapacitors.