The electronic structures and energy band properties of the Cd-doped wurtzite BeO are investigated by plan-wave pseudopotential method with the generalized gradient approximation in the frame of density functional theory. The theoretical results show that the valence band maximum is determined by O 2p states and the conduction band minimum is occupied by Cd 5s and Be 2s orbitals based on the total density of states and partial density of states of Be1-xCdxO alloy. With the Cd content x of Be1-xCdxO increasing, the repulsion effect between Cd 4d and O 2p states is more enhanced and the bandgap of Be1-xCdxO is reduced. At the same time, the bandgap undergoes the direct-indirect-direct transition. In order to obtain the theoretical values in accord with the experimental results, the bandgaps of Be1-xCdxO are corrected. Moreover, the relations among energy bandgap, bowing parameter and lattice constant of the wurtzite BeO-ZnO-CdO ternary alloy are analyzed.