The crystal structure, electronic and optical properties of Cr, O and CrO codoped GaN were calculated by firstprinciple calculation. The results show that codoping method brings in new intermediate band as well as lowering the formation energy in comparison with single oxygen atom doping. The formation of intermediate band enhances absorption of low energy photons, especially in infrared range of the sun spectrum. Our calculation predicts the feasibility of CrO codoped GaN used as a thirdgeneration solar cell semiconductor material.