In this article we have investigated the influence of oxygen adsorption on the surface geometry and electronic properties of Ag(100) surface by using the density-functional theory calculations. The total energy calculations based on projector-augmented wave (PAW) method have been preformed to describe the adsorption geometry at several coverages of oxygen adsorption including p(1×1), c(2×2), and (21/2×221/2)R45°, and a series of main physical properties at these coverages on Ag(100) surface such as the surface relaxation, adsorption energy, work function and so on. The results presented in this work show that for an Ag(100) (21/2×221/2)R45°-2O geometry, the most stable atomic structure is a type of missing-row reconstruction. Eventually, this structural change causes various displacements of surface atoms which have been calculated in this work. The calculations on the local density of states reveal that in the Ag(100) (21/2×221/2)R45°-2O geometry the cohesive effect between the adsorbed oxygen atoms and the substrate Ag layer is essentially due to the sufficient Ag4d-O2p orbital hybridization. Finally we have simulated the STM images for several bias-voltages and tip heights, providing experiments with abundant data and theoretical support.