Ammonium sulfate ((NH4)2SO4) exhibits promoting effects in malachite sulfidization flotation. However, the promotion mechanism remains poorly understood. In this study, micro-flotation tests, zeta-potential measurements, scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and materials studio simulation (DFT) were used to investigated the promotion mechanism of (NH4)2SO4. Micro-flotation test demonstrates that the recovery of malachite from 73% increased to 83%, when the (NH4)2SO4 was added. Contact angle and zeta potential test results indicate that addition of Na2S·9H2O changes the surface properties of malachite and provide the conditions for adsorption of butyl xanthate (BX). After promoting the sulfidization by (NH4)2SO4, BX is more effective in improving the hydrophobicity. SEM-EDS and AFM results show that (NH4)2SO4 can improve performance and stability of sulfidization. X-ray photoelectron spectroscopy indicates that after sulfidization, polysulfides and cuprous were appeared in malachite surface, infers that a redox reaction occurs between sulfur and copper on the surface of malachite. After addition of (NH4)2SO4, the percentage of polysulfides and cuprous were increased, it implies (NH4)2SO4 can accelerate the redox reaction. Computational results show that after adding (NH4)2SO4, the adsorption energy of HS- on the malachite surface is reduced, implies that (NH4)2SO4 can improve the stability of HS-adsorption on the surface of malachite.