Cr-doped and Mg,Cr-codoped Al2O3 crystals were grown by Czochralski method. The latter has a broad absorption peak in 900—1600nm region. We investigated the change of the absorption band with annealing under different atmospheres or temperatures. Through establishing a structure defect model, we successfully interpreted all the experiment results and confirmed that the extremely broad infrared absorption band belonged to Cr4+, which should occupy the octahedral sites in Al2O3.