Malachite is depressed during sulfidization flotation when excess sodium sulfide is added to it. Thus, this study investigated the role and mechanism of ammonium sulfate as a sulfidization promoter for eliminating the depression of excess S2- via micro-flotation experiments, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and inductively coupled plasma mass spectrometry (ICP-MS). Micro-flotation experiments revealed that malachite changed from a non-floating state to an easily floating state by (NH4)(2)SO4 addition prior to sulfidization with xanthate as the collector. The solution chemical reactions involving (NH4)(2)SO4 are the key to re-floating malachite. The surface properties of malachite were changed by (NH4)(2)SO4, and these changes enhanced its flotation behavior. However, (NH4)(2)SO4 was not adsorbed onto the malachite surface in any form, as confirmed by the XPS analysis. (NH4)(2)SO4 has a dissolution on the malachite surface, causing lattice imperfections, which, in turn, provided the conditions necessary for subsequent adsorption of Cu(NH3)(n)(2+), S2- (SH-), and Cu2+ on the malachite surface. Consequently, a layer or multilayer copper sulfide hydrophobic film formed on the malachite surface. Even the low-activity cylindrical surface of malachite was also covered with a stable copper sulfide film, resulting in improved malachite flotation in the presence of excess Na2S.