The possible defect models of Y3+:PbWO4 crystals are discussed by defect chemistry and the most possible substituting positions of the impurity Y3+ ions are studied by using the general utility lattice program (GULP).The calculated results indicate that in the lightly doped Y3+ :PWO crystal, the main compensating mechanism is [2Y+Ppb+ V2-Pb], and in the heavily doped Y3+ :PWO crystal, it will bring interstitial oxygen ions to compensate the positive electricity caused by Y+Pb, forming defect clusters of [2Y+Pb +O2i- ] in the crystal. The electronic structures of Y3+ :PWO with different defect models are calculated using the DV-Xα method. It can be concluded from the electronic structures that, for lightly doped cases, the energy gap of the crystal would be broadened and the 420nm absorption band will be restricted; for heavily doped cases, because of the existence of interstitial oxygen ions, it can bring a new absorption band and reduce the radiation hardness of the crystal.