In order to investigate the properties of the p-type doping and reveal the mechanism of the hole concentration increasing after Cd：O codoping of wurtizite AlN，we have carried out first-principles calculations based on density-functional theory (DFT) for wurtizite AlN system. By calculating the binding energy of the Cdn-O（n=1，2，3，4）complex codoped AlN we found that Cd：O can be stabilized and the solubility of Cd can be increased in the system. We analysed the activation energies of the Cd and Cd2-O doped AlN and found that the activation energy of Cd2-O was decreased by 0.21 eV from that of Cd，which showed that the hole concentration of Cd2-O doped system was raised 103 times as that of Cd. We compared the band structures and densities of states，and found that the electrons of Cd atom on the 4d orbit moving to 2p orbit of N atom results in that the impurity levels which appeared near the Fermi level always occupy the highest valence band. Increasing the number of Cd atoms，the holes can occupy more states. In order to reduce the combining probability of Cd and O，hence，to enhance the Cd-N covalent characteristics and improve the hole concentration，it is important to control properly the concentrations of Cd and O.