In this work, nanoporous ultrafine carbon fibers have been fabricated by one-step activation of electrospun polyacrylonitrile (PAN)/cellulose acetate (CA) ultrafine fiber using ZnCl2 as an activation agent. CA, a renewable resource, was used to replace part of PAN as carbon precursor for electrospun carbon fibers. The effect of additive amount of CA on the microstructure of the ultrafine carbon fibers, including morphology, crystal structure, porosity, specific surface area and surface chemistry, has been investigated. The introduction of CA made an active role in improving structural stability and specific surface area. Both three-electrode and two-electrode test were conducted to evaluate capacitive performance of the ultrafine carbon fibers. And the correlation between electrochemical performance and structure of the carbon fibers was discussed. When CA/PAN mass ratio was 1:9, the ultrafine carbon fibers have the highest specific capacitance of 280 F/g. Such high specific capacitance should be due to combination of high content of surface functionality and wider pores.