Abstract: With the increasing pollution of indoor air of formaldehyde, a simple and convenient adsorption technology is desired. Activated carbon fibers (ACF) is an excellent formaldehyde adsorbent material.Pitchactivated carbon fibers with different surface areas, pore size distribution and surface oxygen-containing groups were obtained by heat treatment and oxidative modification, and were applied to study the adsorption capacity of formaldehyde. The results indicated that P-ACF OG-5A with the smallest BET surface area and pore volume showed the highest adsorption capacity with a breakthrough capacity of 38.73 mg/g. Regression analysis showed that the formaldehyde breakthrough capacity was determined by the surface area and pore volume of ACF with pore size from 0.9 nm to 1.8 nm, instead of BET surface area and total pore volume. Secondly, the formaldehyde adsorption capacity of P-ACF OG-7A was greatly improved after the oxidative treatments by air, hydrogen peroxide and nitric acid. The oxidative treatment by nitric acid (65%) gave rise to the highest adsorption capacity with the breakthrough capacity of 58.21 mg/g, which is 2.5 times higher than that of the sample without treatment. The increased oxygen content in ACF enhanced the formaldehyde adsorption capacity. To further study the mechanism by which the formaldehyde adsorption capacity was increased, Boehm titration and regression analysis were used. The results showed that the numbers of acidic oxygen-containing groups (phenolic hydroxyl groups, lactone groups, carboxyl groups, etc.) in ACF were the main reason that led to the increase in formaldehyde adsorption capacity. The acidic oxygen-containing groups were mainly composed of hydrophilic groups like C＝ O, C−OH which could react with formaldehyde and enhance the formaldehyde adsorption capacity.