In this paper, viscose rayon-based knitted fabrics were utilized as the precursor to produce activated carbon fiber absorbents (ACFA). To obtain better pore characteristics and higher weight yield of ACFA, the effect of flame retardant reagent concentration was studied. Experimental results revealed that both BET surface area and micropore volume increased with increasing flame retardant reagent concentration. On the other hand, both weight yield and micropore volume ratio ($V_{mic}/V_{tot}$) decreased as the flame retardant reagent concentration increased. It was therefore concluded that controlling the flame retardant reagent concentration at 30% not only could obtain better absorption property of ACFA but also helped maintain its production efficiency.