A simple approach to Ag2S quantum dot (QD) modification was used to tune the field emission (FE) properties of ZnO nanowire arrays (NWAs). By a simple and facile successive ionic layer adsorption and reaction (SILAR) approach, Ag2S QDs were uniformly and densely packed on ZnO nanowires (NWs) to form ZnO-Ag2S core-shell heterojunction structures. The FE properties of ZnO NWAs were effectively tuned by controlling the amount of Ag2S QDs. The turn-on field first reduces and then increases as the amount of Ag2S QDs increases, while the trend of the field-enhancement factor is inverse. This is attributed to the clustering of Ag2S QDs into nanoparticles (NPs) which cover the nanowire tips, as SILAR cycles increase.