The effects of Ag doping and defect coexistence on the photoelectric properties of ZnO were studied by first-principles method. The calculation results indicate that O-rich condition is beneficial to Ag doping, while O-poor condition has an inhibitory effect. When Ag doping concentration is lower, AgZn is dominant doping model under O-rich or O-poor conditions, and the model is more stable. When Ag doping concentration is higher, AgZn-AgZn is dominant doping model under O-rich condition, and that is AgZn-Agi under O-poor condition. Under O-rich condition, it is difficult to introduce the coexistence defect of VZn and Oi by Ag doping. VO is a preferred model under O-poor condition, and it can promote Ag doping to a certain extent. Ag doping reduces the band gap of ZnO. The higher of doping concentration is, the narrower of band gap will be. The defect coexistence of VZn, VO and Oi increase the band gap of Ag doped models. Ag doping and the defect coexistence of VZn, Oi all red shift the absorption edge of ZnO to the visible region, which expands the absorption range of ZnO to sunlight. AgZn-VO is transparent in the visible region. In the low energy region, AgZn-AgZn shows higher optical absorptivity, but the corresponding formation energy is also higher than that of AgZn. VZn improves the optical absorptivity of AgZn-VZn and AgZn-AgZn-VZn in the low energy region. VO is beneficial to the adsorption of O2 by ZnO and can produce more H2O2 and ·HO. The defect coexistence of VZn and VO both improve the photocatalytic performance of ZnO.