Using the density-functional theory (DFT) combined with the projector augumented wave (PAW) method, we have investigated the electronic structure of Cd-doped wurtzite ZnO. Analysis of the band structures, density of states (DOS) and partial density of states (PDOS) of CdxZn1-xO shows that the valence band maximum (VBM) is determined by O-2p states and the conduction band minimum (CBM) is occupied by the hybrid Cd-5s and Zn-4s orbital. The energy of CBM decreases and the energy of VBM increases with increasing Cd-doped concentrations. Both effects lead to narrowing of the band gap. Furthermore, it was found that Cd-doped can cause tensile strain in the crystal structure, which also reduces the band gap.