The Cr-doped TiO₂ models were established and the band structures have also been calculated. The similar result is obtained according to the three models: Cr-doping gives intermediate energy of about 1.0eV in the band gap, which means it can shift the absorption edge of TiO₂ to visible region .But too much Cr-doping can also result in recombination center of electron-hole and a decline in photocatalytic activity. Nanocrystalline Cr-doped TiO2 powders with different initial Cr/Ti ratio in starting materials have been prepared by hydrothermal method. The results show that the prepared powders are all composed of anatase TiO₂, the BET specific surface area of the powders are ranged form 178 to 221 m2/g, the averaged grain size is about 9 nm and the size distribution is narrow. Cr ³⁺ has been doped into the lattice of TiO2 instead of Ti. All the absorption edges of Cr-doped TiO₂ powders with different nominal doping concentration of Cr ³⁺ have been red-shifted into visible light region. In our case, Cr-doped TiO₂ with initial Cr/Ti atomic ratio of 2% in starting materials has the best photocatalytic activities when decompose the MB in its aqueous solution, which is consistent with that of the theoretical results.