Abstract: Polyacrylonitrile-based preoxidized nanofiber mat was microwave carbonized in nitrogen atmosphere for preparing carbon nanofiber mat，then water steam was taken as activating agent for preparing the activated carbon nanofiber mat. The operation conditions were optimized through orthogonal experiment. Field emission-scanning electron microscope，infrared spectrometer，X-ray diffraction were employed to characterize the morphology and structure of the carbon nanofiber mat. Specific surface area，pore volume，pore size distribution and phenol adsorption value of the obtained carbon nanofiber mat were measured to evaluate the effects of the operation factors in the orthogonal experiment on the yield of activation and adsorption properties of activated carbon nanofiber mat. Results showed that activated carbon nanofiber mat with excellent adsorption properties could be prepared under the optimized operation conditions. The obtained carbon nanofiber mat showed mainly micropores with specific surface area of 1107.4 m2/g and phenol adsorption value of 428.1 mg/g.