Abstract: In order to investigate the microstructure variation of phenol liquefied wood carbon fibers caused by activating treatment by Na₂HPO₄, by using Na₂HPO₄ solution as the activator, the phenol liquefied cunninghamia lanceolata carbon fiber precusors were impregnated, dried and activating treated at different temperatures. The crystal structure, pore structure and surface chemical structure of activated carbon fibers were characterized. The results show that the yield of activated carbon fibers decreases gradually with the increasing of activating temperature. The crystal structure of activated carbon fibers belongs to graphite-like structure. With the increasing of activation temperature, the microcrystalline layer spacing d₀₀₂ decreases, while plan size of graphene sheets L_c and L_c/d₀₀₂ increase. When the activation temperature is 600 ℃ or 700 ℃, the micropore ratio is below 48vol%; when the activation temperature is 800 ℃ or 900 ℃, the micropore ratio is above 60vol%. The microporous pore size of activated carbon fibers mainly concentrates in the range of 0.5-1.6 nm, and the mesoporous pore size mainly distributes in the range of 2.0-4.0 nm. With the increasing of activation temperature, the specific surface area and pore volume of fibers gradually increase, and both reache the maximum at 900 ℃, the specific surface area is 1 306 m²/g in this case. C and O are the basic elements of activated carbon fibers. On the surfaces of fiber, a large proportion of carbon-containing groups are graphite carbon, and containing a small proportion of C-OH, C=O and -COOH. The investigation provides references for preparing new type activated carbon fibers and further ascertaining the interaction between activator and molecules of carbon fibers.