We studied the isothermal adsorption of naphthalene on a viscose-based activated carbon fiber (VACF) in aqueous solution. The adsorption experimental data were fitted to adsorption equations such as the Langmuir,Freundlich,Dubinin-Radushkevitch(D-R),Dubinin-Astalov(D-A),Langmuir-Freundlich(L-F),and Redlich-Peterson (R-P) equations. The effects of temperature,pH,and pre-loaded copper ions on the VACF on the adsorption of naphthalene were investigated. The fitting results show that the micropore volume filling theory fits better than the single-layer adsorption theory for naphthalene adsorption onto VACF. The adsorption of naphthalene occurs spontaneously between 20 and 40 ℃. The adsorption capacity of VACF for naphthalene decreases significantly above 30 ℃. A new mechanism is proposed to explain why the adsorption capacity increases significantly in lower pH aqueous solutions. The reduction of naphthalene takes place on the VACF with pre-loaded copper ions.