This paper proposed a promising method of improving the sulfidization flotation of smithsonite through surface modification with ammonia. The floatability of ammonia-modified smithsonite was superior to that of bare smithsonite under the same flotation condition due to the enhanced sulfidization. Results of zeta potential and surface adsorption experiments showed that the preliminary modification of smithsonite surfaces with ammonia increased the amount of sulfide ion species adsorbed onto the mineral surface, thereby facilitating the attachment of xanthate species onto the sulfidized mineral surface and promoting its hydrophobicity. XPS measurement revealed that surface modification with ammonia prior to Na2S addition increased the amounts of zinc sulfide species formed on the mineral surface and altered the chemical environment of sulfidization products. Furthermore, the activation mechanism of ammonia modification for enhancing the adsorption of sulfide species onto the smithsonite surface was studied by solution chemistry calculations. Zinc ammonia complexes were found to interact with hydrophilic species coated on the smithsonite surface. This interaction increased the number of zinc sites on the surface of ammonia-modified smithsonite and thus enhanced its sulfidization flotation.