Semiconductor photocatalysts have received much attention due to their applications of wastewater treatment and air purification. The monoclinic -AgVO3, which has narrow band gap (2.11 eV) and can respond to visible light, has been considered as one of the promising semiconductor photocatalysts. The vacancy defects always exist in -AgVO3 prepared under the conventional synthesis conditions and have important influences on the structure and properties of -AgVO3. Systematic theoretical study of the vacancy defects in -AgVO3 is still lacking. In this paper, using density functional theory plus U (DFT+U) approach, the Ag vacancy, O vacancy and Ag-O bivacancy in -AgVO3 are studied. The formation energy, band structure, differential charge density and optical absorption spectrum of -AgVO3 with vacancy defects are carefully investigated. When the U values are chosen as 6 eV and 2.7 eV for the Ag-4d and V-3d electrons respectively, the reasonable lattice parameters and band gap value can be obtained for -AgVO3. By comparing the formation energies of different Ag and O vacancies, we find that the dominating vacancy defects in -AgVO3 are Ag3 and O1 vacancies, and the formation of Ag vacancy is much easier than that of O vacancy. The analyses of the total and partial density of states indicate that the conduction band arises mainly from V-3d orbit, and the valence band is mainly composed of Ag-4d and O-2p states for -AgVO3. With Ag3 vacancy, O1 vacancy or Ag3-O1 bivacancy, the band gaps of -AgVO3 all decrease in different degrees. The Ag3 vacancy behaves as p-type donor, allowing the Fermi level to shift down to the valence band maximum. However, O1 vacancy and Ag3-O1 bivacancy both act as n-type donors, and the Fermi level shifts to the conduction band minimum. The change of the Fermi level for the vacancy defect systems also means that the charge transfer occurs among the atoms around the vacancy, which is analyzed by calculating the differential charge density. The Ag3 vacancy and O1 vacancy have little effects on the light absorption of -AgVO3 in the range of visible light, while O1 vacancy and Ag3-O1 bivacancy in -AgVO3 cause the obvious absorption of light in the near infrared region.