The existing forms of the impurity Nb 5+ ions in the Nb:PWO crystals are simulated by the computer simulation technology. The various kinds of defects and substitution reactions in the Nb:PWO crystals are also simulated. By analyzing the calculated results of defect formation energies and solution energies, the optimal positions of the Nb 5+ ions and the charge compensating mechanism [NbO 3 +V O ] + –[NbO 4 ] − in the Nb:PWO crystals are obtained; this charge compensating mechanism change the compensating form of oxygen vacancies which relates to 350nm absorption band, then the 350nm absorption band is depressed.