The band gap width of ZnO/GaN heterojunction is large, which restricts absorption of visible light. In order to study the effects of Ag on the visible light absorption of ZnO/GaN heterojunction, GaN/ZnO heterojunction is constructed on the (1-100) nonpolar plane, and Ag is utilized to replace Zn and Ga atoms at different positions, respectively. Then, the influence of Ag doping on the stability, electronic structure, optical properties and band edge position of ZnO/GaN heterojunction is calculated by first principle. The results reveal that the system of Ag doped ZnO/GaN heterojunction is stable because of the negative formation energy. For Ag replaced Zn and Ga, the band gap width decreases from 2.93 eV to 2.7 eV and 2.3 eV, which results in red shift of absorption coefficient as well as photoconductivity, and is beneficial to the absorption of visible light. Finally, Ag doped ZnO/GaN heterojunction has excellent photocatalytic activity.