The positions of the interstitial oxygen atoms in PWO crystals are numerically simulated by. It was found that the formation energy of isolated point defect is small when oxygen atom exists around the (WO4)2-. Based on the computed results, the total density of states and partial density of states of perfect PWO crystals and PWO crystals with interstitial oxygen atoms were calculated using DV-X α program. It is concluded that interstitial oxygen atoms will combine with oxygen ions of (WO4)2-, forming oxygenic molecule ions which are related to the 350 nm absorption band.