Activated carbon fibers (ACFs) with different pore structure have been prepared from wood sawdust using the KOH activation method. A study was conducted to examine the influence of the activation parameters (temperature, alkali/carbon ratio, and time) on the morphology and structure of the as-prepared ACFs developed in the process of pore generation and evolution. Activation temperature was very essential for the formation of utramicropores (<0.6 nm), which greatly contributed to the electric double layer capacitance. The significance of metallic potassium vapor evolved when the temperature was above 800 °C, since the generation of 0.8- and 1.1 nm micropores cannot be ignored. When the the KOH/fiber ratio was increased and the activation time was prolonged, to some extent, the micropores were enlarged to small mesopores within 2–5 nm. The sample with the optimal condition exhibited the highest specific capacitance (225 F g–1 at a current density of 0.5 A g–1). Its ability to retain capacitance corresponding to 10 A g–1 and 6 M KOH was 85.3%, demonstrating a good rate capability. With 10 000 charge–discharge cycles at 3 A g–1, the supercapacitor kept 94.2% capacity, showing outstanding electrochemical performance as promising electrode material.