•Possible reaction paths for the preparation of ACFs from liquefied wood were proposed.•Some incompletely liquefied substances decomposed during carbonization.•Reactions between KOH and carbon began at 530°C.•The main gaseous products were H2, H2O, CH4, CO, CO2. In this study, wood-based fibers were used as raw materials to prepare activated carbon fibers (ACFs) by carbonization at 500°C followed KOH activation at 850°C process. The structure of the fibers was characterized by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analyses. Besides, the volatiles of the final products were determined by thermogravimetry-mass spectrometry (TG-MS) and titration. The reaction mechanisms were consequently deduced. In the first carbonization process, it was mainly related to substitution, scission, and oxidization reactions of methylene. And in the second activation process, KOH and carbon began to react at 530°C, resulting in potassium compounds, which further reacted with carbon. Meanwhile, temperature elevation caused polycyclic reactions, which made the ACFs a graphite-like microcrystalite structure.