Bis(acetylacetonato) iron(II) (Fe(acac)₂) was first selected to mediate the reversible-deactivation radical polymerization (RDRP) of acrylonitrile (AN) with predictable molecular weights, relatively narrow molecular weight distributions and high end-fidelity. The reversible termination (RT) mechanism accounted well for the reaction phenomena. The spinning performance of polyacrylonitrile (PAN) was investigated and the average diameter of the spinning fibers was approximately 300 nm. Activated carbon fibers (ACFs) were prepared via sodium hydroxide (NaOH) direct activation. The maximum specific surface area of the ACF was as high as 1165 m² g⁻¹. The pore volume of the ACF reached 0.14 cm³ g⁻¹. Most is the contribution of the small mesopores (2–5 nm). The unique microstructures enabled the ACFs to have good compatibility with potassium hydroxide (KOH) as electrolyte. The highest capacitance reached 167 F g⁻¹ at room temperature. The electrode showed well stabilized capacitance after 500 cycles and high energy/power density values due to the formation of small mesopores.