Cr-doped TiO₂ nanoparticles were synthesized by one-step flame spray pyrolysis technique. The effects of Cr³⁺ doping on the microstructure, light absorption property, and photocatalytic activity of the samples were investigated. It is found that increasing Cr³⁺ concentration restrains the formation of anatase and simultaneously favors the formation of rutile. At low concentration (≤1%), Cr is mainly incorporated into the crystal lattice of TiO₂ in the form of Cr³⁺, while high Cr concentration promotes the formation of Cr₂O₃ clusters. The result of photocatalytic degradation of 2,4-dichlorophenol demonstrates that appropriate Cr³⁺ doping can evidently enhance the visible-light photocatalytic activity of TiO₂. The optimal Cr3+ concentration to obtain the highest photocatalytic activity is 1at%. The improvement of visible-light photocatalytic activity derived from Cr doping is mainly related to the enhancement of visible light absorption, appropriate crystalline composition, and improved transfer efficiency of photogenerated electrons and holes.