Activated carbon fibers (ACFs) with high adsorption performance and mechanical strength are one of the essential materials to meet the needs of industrial gas adsorption. The activation step is the most critical step affecting the performance of ACFs. This study presents a facile one-step carbonization and activation method for preparing activated carbon fibers with a phenolic fiber as a carbonaceous precursor, followed by CuCl2 as an activating agent. The prepared activated carbon fiber (ACF-0.1) has three main advantages that are high microporosity of 89%, less use of an activator (mass ratio of PF and CuCl2 was 1:0.1), and excellent mechanical properties. In particular, taking advantage of ultra-micropores (<1 nm) created by low-dose CuCl2, activated carbon fibers can be used well as solid adsorbents for the CO2 capture, which show a high selectivity of 22.3 (calculated by LAST) and valid separation performance at 298 K and 0.15 bar. Meanwhile, the activated carbon fibers with developed micropores created by high-dose CuCl2 exhibit an active role in gas-phase toluene adsorption. In addition, the activated carbon fibers have considerable flexibility and outstanding mechanical strength, which is nearly 3 times as high as the commercial rayon-based fibers. This method is significant in producing high-performance activated carbon fibers for adsorption in practical engineering applications.