Nowadays, the studies on absorption spectra of Ag-doped ZnO have given two distinctly different experimental results, that is, the red shift or blue shift when the mole fraction of the impurity increases in a range from 0.0278 to 0.0417. To solve this contradiction, according to the first-principles plane-wave pseudopotential of the spin-polarized density functional theory (DFT), we set up three models for Zn1-xAgxO (x=0, 0.0278, 0.0417) to calculate the geometric structure and energy via the method of generalized gradient approximation (GGA+U). Calculated results indicate that compared with the Zn-O bond in pure ZnO system, the value of population decreases, and the bond length of Ag-O in Ag-doped ZnO system increases, this means covalent bond weakens and ionic bond strengthens. With the mole fraction of impurity increases in a range from 0.0278 to 0.0417, the orbital charges of O-2p, Zn-4s and Zn-3d keep unchanged, while the orbital charge of Ag-5s increases, and that of Ag-4d is reduced; the volume and total energy of the doped system increases, causing the system more unstable. Moreover, the formation energy of the doped system becomes lower, thereby making the doping difficult. Meanwhile, the band gap in the system narrows, and its absorption spectra exhibits a redshift. The calculated results are consistent with the experimental data, and can explain the phenomena reasonably. These results may be used in future design and preparation of new type photocatalyst from Ag-doped ZnO as a theoretical basis.