Cu-doped cellulose nanofibers/nano-titanium dioxide (Cu-TOCN/TiO2) photocatalysts were prepared by the hydrolysis-precipitation method using TiCl4 as the source of titanium and cellulose nanofibers suspension as a reaction medium. The prepared photocatalysts were used to decolorize organic dyes (reactive brilliant red K-2BP and cationic red X-GRL) efficiently under the synergistic effect of simultaneous adsorption and photocatalysis. The combination of TOCN inhibited the growth of TiO2 crystals, reduced agglomeration and increased the specific surface area. When compared with TiO2, TOCN/TiO2 improved the decolorization efficiency of the two dyes by 14.82% and 22.87%, respectively, under UV-light irradiation. The absorption edge exhibited red-shift from 380 to 410 nm after Cu doping. An excellent photocatalytic activity was recorded by 0.5 mol % Cu-TOCN/TiO2 and the decolorization efficiency of the two dyes was further improved by 34.76% and 10.44% respectively, compared with no Cu doping. After 2 hours of irradiation, the decolorization efficiency reached 96.57% and 99.73% respectively, while under dark conditions, it was 47.64% and 91.56% for the two dyes. The degradation mechanism of the dyes was verified as the destruction of the azo chromophore and benzene ring. This work provides a potential method for the development of a novel adsorptive photocatalyst with excellent recyclability.